The former deputy leader of the party said constantly depicting the UK government as an “anti-Scottish institution” hardens the minds of No voters, as does regarding Scottish supporters of the union as “traitors”.
His hard-hitting criticisms of SNP policy come as former SNP MSP, and author of the party’s Growth Commission report on the economics of independence, Andrew Wilson, said an early push to establish a separate Scottish currency if the country were to become independent could lead to it being devalued and risk “capital flight”.
The double whammy of warnings to the SNP come just days after Nicola Sturgeon reaffirmed her commitment to holding a second independence referendum in the first half of the new parliamentary term.
At the announcement of her draft co-operation agreement with the Scottish Greens, she said the deal made it “impossible on any democratic basis” for Boris Johnson to continue to refuse a second vote on the constitution.
According to Mr Sillars in his autobiography, "A Difference of Opinion: My Political Journey”, continually suggesting that Westminster was trying to “shaft” Scotland, was not helpful to the independence cause.
"That fires up the party but does it serve the nation's interests? I think not,” writes Mr Sillars. “I am not sure that it serves the independence movement well either."
He adds: "The fact is no one sits in ministerial offices in London thinking of how to shaft Scotland.
"At the very worst, they simply don't give us much thought. That idea that deliberate "Scotland shafting" is going on has a grip on the 'Yes' and SNP minds and it has led to the policy of advancing the case for independence through manufactured grudge and grievance.
"The 'they are taking back powers' rage is an example. Grudge and grievance complaints against Westminster, combined with vitriolic attacks upon unionists via social media, make for a stupid policy. If 'Yes' is to win next time, it will be on the basis of persuasion and that will require a better understanding as to why so many No voters held to the Union last time and still do."
In the Sunday Times, Mr Sillars also warned the SNP approach gives a voice to “anti-Englishness” in Scotland.
"I don't think we should exaggerate the undertone of anti-Englishness that exists in Scotland, but it is there and the grudge and grievance tactic of the SNP leadership does play to it and keeps it alive," he said.
Mr Sillars also believes that by not accepting the result of the Brexit referendum, the SNP leadership undermines it’s own case for independence.
"By leaping upon every anti- Brexit forecast by the Treasury, Bank of England, Institute for Fiscal Studies and English-based think tanks, the party will find it impossible to reject their damning forecasts of the consequences of independence next time, as all such bodies did last time."
Meanwhile Andrew Wilson, who led the party's economic growth commission for Nicola Sturgeon, warned SNP members that moving too quickly to a separate currency in an independent Scotland could see the value of it collapse, hitting incomes and pensions, with the added risk of "capital flight" – money leaving the country.
Mr Wilson backs the SNP leadership view that an independent Scotland should initially keep the pound and allow the Bank of England to retain sovereignty on monetary issues.
However he added: "It is going to be hard work but it will ultimately be worth it."
Speaking on the Scotland's Choice podcast, Wilson added: "We want to manage for both the economic and political uncertainty that would come if we were going to move too quickly on currency.
"The risk would be that the currency would come into being and then quickly devalue, which would be most people in the markets' expectation.
"That would have an effect on people's income if they were waged in Sterling, it would affect people's pensions and mortgages.
"It could be upside, it could be downside; the key thing is it would be uncertain and the thing we want to manage for is the risk of money leaving the country, capital flight which for a new nation is a major risk."
The Scottish Labour MP Ian Murray said Wilson's comments started “to articulate the catastrophe of the SNP's plans but barely scratched the surface on the reality that would be faced by all Scots".
The SNP has been asked for comment.