Abstract: The factors to effect change of free height of steel 44Si2CrV spring during long time loading for 72 h under stress to be lower than yield strength of steel has been studied. The results show that the reduction of free height of steel 44Si2CrV spring under compressive stress 900 MPa consists of two parts: plastic deformation of surface decarburized zone to hinder elastically deformed internal material recovering, making up 84% of total reduction of free height and creep of steel at ambient temperature making up 16% of total reduction of free height.