The Tory politician pilloried the EU chief after an intense week of talks between Brexit Secretary David Davis and Brussels negotiator Michel Barnier.
Mr Rees-Mogg said: “The position the EU has set out that it should have money first is a completely unreasonable one.
“This stance comes very directly from the Commission.”
Speaking on the BBC’s Any Questions programme, the man defended his colourful comments against Mr Juncker.
He said: “Somebody pointed out to me that I was being a bit hard on pound shop and I said that Mr Juncker was a pound shop Bismarck.
“I think in politics using colourful language helps get arguments across: we are discussing this issue because of the language that has been used.”
Mr Rees-Mogg blasted the European Union for demanding a substantial pay-out bill in the region of £92billion to be paid before the UK leaves the EU bloc.
The Commission argues Britain should pay the amount before any discussion on the future relationship between the sides starts.
However, Mr Rees-Mogg rebuked the demands: “If a country were leaving the EU that was a net recipient of EU funds, would the EU be saying that we should agree what we will pay this country when it leaves as the opening step?
“Can you imagine that that is how they would begin their negotiation? Once you’ve thought about that you’ll realise how unreasonable their stance is.”
The Tory MP for North East Somerset said the UK has a strong position and is not legally required to pay further contributions to Brussels.
He said: “Our position is very strong as a very important House of Lords report that looks at international EU and UK law on our contributions once we leave and under all three, international EU and UK law, and Article 50, we have no continuing contribution.”