The city’s rainy day fund is at risk of drying up, fiscal watchdogs warned Wednesday.

The budget surplus plummeted by $355 million over the past year from $4.221 billion to $4.576 billion, according to City Comptroller Scott Stringer.

Those funds go into the city’s budget cushion — reserves, prepayments and retiree health benefits — that can be used if the economy crashes.

“We not only didn’t add to our budget cushion we actually reduced it. That’s the first time this has happened since 2014,” Stringer said at a state Financial Control Board meeting. The cushion dipped from 11.3% to 10.7% of spending.”

State Comptroller Thomas DiNapoli also chided the mayor, who is on the board and was at the meeting, for not tightening the city’s fiscal belt.

“Spending has grown relatively rapidly in recent years and although it is currently affordable it may not be sustainable during a prolonged economic downturn,” DiNapoli warned.

Mayor Bill de Blasio insisted the city’s reserves on a percentage basis are up over the past six years and pledged to continue cutting spending at agencies.