The invention discloses a fixed-time pressure sealing method for a foam mould. The fixed-time pressure sealing method comprises the following steps: filling a foamed polyurethane material into a foamed foam mould; after closing upper and lower moulds of the foam mould, introducing compressed air into an air pocket with a low-pressure value to push the lower mould to close due to the fact that the foam mould adopts lock mould-locking pressure when internal pressure of a mould cavity at an initial stage of a polyurethane chemical reaction is smaller than 2bar, wherein a gap of 0.01-0.05mm is formed between an upper mould mouth and a lower mould mouth of the foam mould, and the gap of 0.01-0.05m replaces a gas exhaust slot to realize gas exhaustion; after carrying out gas exhaustion by set time, increasing internal pressure of the mould cavity to 2-3 bar along with continuing of chemical reaction, switching the mould-locking pressure of the foam mould until the foam mould is completely sealed. According to the fixed-time pressure sealing method, the mould-locking pressure of the foamed foam mould is increased grade by grade, the gas exhaustion slot is required to adopt while the gas exhaustion function is realized better, so that no-flash foams are obtained. Polyurethane raw materials can be saved by more than 1%, and labor power can be saved more than 10%.