Mr Sillars will accuse Better Together of "peddling a fraud, led by a man whose economic judgment is flawed," as he addresses a conference of left-wing supporters of independence in Edinburgh.
"Until now, mistakenly in my view, the Yes side has given Better Together and its leader an undeserved easy ride," he will say.
"It's time for the truth. Better Together is peddling a fraud, and its leader needs to be reminded why his claim to judgment on economic issues is a bad joke.
"With their slogan of Better Together, you wonder if that lot live in the same country as the rest of us."
Mr Sillars, a former deputy leader of the SNP will say: "Why anyone should listen to Alistair Darling is a mystery. He was complicit in every disaster the Blair-Brown government engaged in. As Chancellor, his 2008 Budget speech claimed growth would rise to 3% by 2010, while outside his fantasy world the financial tsunami was halfway across the Atlantic to break upon our economy.
"Read that speech. Every judgment is wrong. He told the House of Commons: that 'the British economy will continue to grow through this year and beyond', and then boasted 'we have maintained confidence and stability in the banking system'. He hadn't. It collapsed.
"Darling is the last man to lecture us on our competence to run the Scottish economy."
A spokesman for Better Together responded: "Rather than debate the issues, or recognise that the nationalist case for breaking up the UK has collapsed around their ears, leading nationalists now want their supporters to launch personal attacks on people who they don't agree with. How depressing."