The Fraser of Allander Institute takes issue with finance secretary Derek Mackay’s approach to presenting his figures and is calling for a change.
But the think-tank has found that Holyrood has suffered a cut in its “discretionary” spending power for areas like schools and hospitals.
In a blog post, the institute said: “The information in the budget isn’t particularly easy to understand.
“And the selective data that the government presents often appears designed to support their arguments rather than to help inform debate.
“An improved budget process is a necessity. With the Scottish budget now dependent not simply on a block grant from Westminster but on devolved taxes, debates are only going to get even more complex. We urgently require a process that is more fit for purpose.”
It continued: “The draft budget document is confusing and overly complex. It is difficult for even seasoned public finance professionals like ourselves to follow. The budget document needs to be clearer on the split between factual information/data and policy/political interpretation.
“At the moment it is a muddle between a policy document and the authoritative source of budget data.”
Mr Mackay has come under fire over a £327 million cut to direct council funding in next year’s budget – and claims other sources mean town hall budgets will rise by £240m.
The finance secretary clashed with Tory finance spokesman Murdo Fraser over the Scottish Government’s funding from Westminster this week.
Mr Mackay insisted this is falling by 9.2 per cent, while the Tories insist it has gone up.
The institute says the best way to measure spending is on the amount from capital and revenue budgets, as well areas of annually managed spending, like business rates, which ministers can use.
“On this measure the budget has declined by around 5 per cent in real terms since 2010-11,” it states.
“As a result of constitutional reform, added to this should be the new capital borrowing powers – which would lead to the discretionary element falling by 3.8 per cent.”
Mr Fraser said: “Derek Mackay should be embarrassed at these damning criticisms of the way in which he tries to spin his budget.”