In an interview with The Herald, the Chief Secretary to the Treasury insisted the UK Government's series of analysis papers were cutting through to the public and exposing the Nationalists' "rickety" prospectus.
He countered the Yes campaign's claims of perpetual scaremongering by the No camp, saying it was not negative to ask the SNP "simple, basic questions" about their proposals.
"The Nationalists may have spent 80 years thinking about those sorts of questions and have an answer to them. Astonishingly, it seems they have spent 80 years believing in independence but not really thinking about what it would mean or how it would work.
"It is pretty extraordinary the SNP is coming out and trying to sell independence on this rickety basis."
Mr Alexander claimed the Whitehall papers have at their heart a "positive case" for the Union on the economy, trade and the currency.
"The moment you ask someone a reasonable question and they go - oh my goodness, you're scaremongering, it's Project Fear, you know they haven't got an answer.
"The SNP don't have the answers. They know they don't have the answers. The UK Government papers are presenting a weight of evidence and by the time it's finished you'll have 1000 pages of evidence the SNP will need to answer if they are going to convince anybody."
The deputy Chancellor insisted the analysis papers were presenting the "evidence and facts behind what the UK Government believes - that the United Kingdom is the most successful alliance between countries ever in world history".
Mr Alexander referred to the latest paper, which suggested that creating a border between Scotland and England could cost every Scottish household £2000 in lost income after 30 years. But the Chief Secretary stressed this was a conservative figure and it could be much more.
"These numbers are on absolutely the best-case scenario for independence. The £2000 figure is a prudent, conservative figure.
"I have said it is highly unlikely that an independent Scotland will be able to have a monetary union with the rest of the UK and keep the pound, but one of the assumptions in the analysis was Scotland would be in a monetary union with the rest of the UK.
"So it was on the most favourable possible terms and so the £2000 figure is a very modest estimate and the more barriers there are in the way the worse it gets," he added.
Meanwhile, Willie Rennie, the Scottish Liberal Democrat leader, warned the SNP of placing too much emphasis on the issue of different identity and values and said it was dangerous to suggest those against independence were not true Scots.
"Of course there are differences in Scotland, but I don't think we are uniquely different," he told a conference fringe.
"To force people to choose is the most insidious part of this referendum, which is why the Nationalists have to be careful. The leadership are trying to keep a lid on it. Occasionally, it slips.
"To try to say that we all have to believe in independence or we are not Scottish is a dangerous thing to do."
In response, SNP MSP Stuart McMillan, said: "Mr Rennie doesn't seem to understand what the referendum is about; it's about democracy, not identity.
"We have a wide range of identities in modern Scotland - Scottish, British, English, Pakistani, Chinese, Polish, French, Irish and many, many more. Independence is the broad, inclusive and positive choice for Scotland, in which all these identities can be reflected and celebrated," he added.