Mr Johnson has been under fire in recent weeks over a number of issues, including the recent announcement by the Electoral Commission of an investigation into how the refurbishment of his Downing Street flat was funded.
The former Scottish first minister questioned whether Mr Johnson will still be in office in two weeks, adding that he does not believe the Prime Minister will be as steadfast against granting another independence referendum as he has made out.
Mr Salmond told STV: “People keep telling me what an immovable object Boris Johnson will be. I think a puff of wind would blow him over at the present moment.
“I’m not even sure he’ll be in Downing Street in two weeks’ time.
“I don’t think moving Boris Johnson on the independence referendum is going to be as difficult as people feel it is, but if you don’t start that process you’re never going to finish it.”
Mr Salmond, who said his party was launched to inject “urgency” into the independence debate, refused to say what other measures he would advocate if a referendum is not granted.
The Alba Party and Mr Salmond have said the Scottish Government should negotiate with Westminster on how best for Scotland to become independent, and say it should not limit its ambition to holding a referendum.
Mr Salmond said: “You should keep your options open – you don’t set your ambition for a Section 30 referendum. That’s just an invitation for Whitehall to turn it down.
“In a negotiation, you don’t always tell the other side what your ambitions are. That’s an invitation for them to say no to you.
"You need to have other things up your sleeve – plan B as it’s sometimes called. I think you need plans C, D and E as well when you’re facing down Westminster.”
The former first minister’s comments come after Ofcom on Wednesday dismissed complaints about the BBC's coverage of the Scottish Parliament election after Alex Salmond complained of a “virtual blackout” of his party by the broadcaster.