The present invention relates to a foaming composition of soft polyurethane used for the cushion insert for vehicles and more specifically, to a foaming composition of soft polyurethane which comprises polymerpolyol wherein solid polymer synthesized from acrylonitrile or styrene monomer is grafted or dispersed in polyetherpolyol, polyol wherein polyetherpolyol is mixed, crosslinkage agent, amine catalysts, foaming agent and organic silicon defoaming agent, and which provides greatly improved productivity and reduced inferiority owing to the natural crushing of independent foam cells through chemical method rather than physical method.