PURPOSE: To make it possible to prepare a polyurethane resin of low hardness with good productivity, by adding a specified catalyst in preparing the polyurethane resin from a long-chain polyol, a diisocyanate and a low-MW diol in an extruder by a one-shot process.

CONSTITUTION: In preparing a polyurethane resin by reacting a long-chain diol (A) such as polytetramethylene glycol, a diisocyanate (B) such as diphenylmethane 4,4'-diisocyanate and a low-MW diol (C) such as 1,4-butanediol in an extruder by a one-shot process, 0.001W3 pts. organometallic catalyst such as dibutyltin dilaurate or lead octenoate, tert. amine catalyst such as triethylenediamine, stannous salt such as stannous chloride based on 100 pts.wt. resin are added and the reaction thereof is performed. The rate of the synthetic reaction can be thereby easily adjusted and especially, synthesis of polyurethane of low hardness can be realized in the extruder.