He was speaking after his UK counterpart, David Davis, made clear the threat to walk out was genuine if the EU's "divorce bill" was too high.
EU ministers on Monday gave Mr Barnier the green light for talks to start in June, after the UK election.
"No deal" was not an option, he said.
UK ministers have reacted angrily to reports that the EU may demand as much as â‚¬100bn (Â£86bn; $112bn) from the UK.
The EU is also insisting that "sufficient progress" be made on the bill, citizens' rights and the UK-Irish border before talks begin on a future trade deal.
The first round of talks will start on 19 June and Mr Barnier will report to EU leaders at a summit three days later. In a communique on Monday, EU officials stressed that a key to the talks' success would be their transparency for all sides.
UK Prime Minister Theresa May emphasised the importance of the negotiations in the days after voters choose a new government on 8 June. "There will be no time to waste and no time for a new government to find its way," she said.
"If we don't get this right, the consequences for the United Kingdom and for the economic security of ordinary working people will be dire. If we do, then the opportunities ahead are great."
Mr Barnier told reporters in Brussels that the facts and figures behind Brexit had to be explained objectively, and it would not be "business as usual".
"We just need to be able to wind up the accounts - that's it. It's really a question of trust to build our future relations."
A range of figures has emerged for the amount the UK will be asked to pay when it leaves the EU, covering agreed commitments and liabilities.
Asked a second time what he would do if the British walked out of the talks, Mr Barnier said there would be moments of tension but things had to be put in perspective.  "The new partnership is what's important. Nobody should lose that perspective," he emphasised.
The European Union's chief Brexit negotiator, Michel Barnier, has said he does not want to consider the chance that talks on the UK's exit from the EU could collapse.