Method for improving storage stability of polyurethane prepolymer

The invention provides a method for improving the storage stability of a prepolymer by adding an effective amount of an additive to the system, the additive comprising a) a phosphate ester and b) a silane coupling agent. The phosphate ester is selected from at least one of compounds represented by chemical formulas (I), (II) and (III): in the formula (I), R1 and R2 are the same or different and are respectively a hydrogen atom, an alkyl group or an aryl group, and at least one of R1 and R2 is an alkyl group or an aryl group; in formula (II) and formula (III), R3 and R4 may be the same or different and each represents a hydrogen atom, a hydrocarbon group, or an aryl group, at least one of R3 and R4 represents a hydrocarbon group or an aryl group, AO represents an alkylene oxide group, and n represents an integer of 1 or more. And the silane coupling agent is at least one of KH550, KH792 and KH602. The additive disclosed by the invention can stably exist in MDI and plays a role in synthesis and storage processe