Alistair Darling has been optimistic on the prospects for economic growth and the amounts raised from the new 50 pence top rate of income tax, MPs have said.
The Commons Treasury committee also criticised the lack of a formal structure to constrain government borrowing.
In its critical report on the 2009 Budget, published on Wednesday, the committee also called for clarity on how the government commitment to halve child poverty by 2010/11 can now be reached.
The MPs said that it was "an optimistic assumption" to believe the Treasury's growth forecasts could be met amid continuing economic uncertainty.
Committee chairman John McFall said: "We are not convinced that the Budget forecasts fully acknowledge this uncertainty.
"We all want to see a way out of recession, but we need to be realistic," he added.
The report also warned that "there are considerable uncertainties over the yield to be raised by the 50 per cent top rate of income tax".
It called on the Treasury to report on the revenue raised by the move, both nominally and as a percentage of the theoretical maximum revenue.
And the cross-party committee also warned that the credibility of the public finances "will depend on an acceptance that the structural deficit must be addressed as well as the consequences of the current extraordinary circumstances".
It added that the 'temporary operating rule' - being used by the Treasury following the abandonment of the fiscal rules used when Gordon Brown was chancellor - appears to offer "no constraint at all" on fiscal decisions.
McFall said: "It is critically important that both the public and financial markets believe that the chancellor is working to an adequate plan to restore the public finances to good health.
"Accountability to Parliament and proper public consultation are more important than ever in restoring confidence."
The committee also said it was "unconvinced" that schemes to boost the housing market, such as the stamp duty holiday, will have any marked effect.
