Salmond, 61, who led the Scottish National Party and was first minister of Scotland before resigning when independence was defeated 45% to 55% in a 2014 vote, is campaigning against a so-called Brexit .
Opinion polls show Britons are divided on whether to remain in the 28-member European bloc or not.
Salmond, who is now a lawmaker in the London parliament, said it could come down to the wire.
"I think the referendum will be a damn close-run thing," he told AFP, criticising Conservative Prime Minister David Cameron's campaign to remain within the EU as "negative".
"The approach of the Prime Minister... is to make essentially a negative case, a scaremongering case," Salmond said.
"The problem with that case is essentially it's a lie. Yes, of course, leaving the EU will cause economic difficulty, trouble, tribulations, but it's not a disaster. And it doesn't signal the end of international trade."
"The right campaign in order to engender the enthusiasm of Europe is to say: this is what we think Europe should be doing.This is the Europe we can build, the Europe we can seek."
Salmond warned that it could all come down to differences in opinion between Scotland and the rest of the United Kingdom.