The main occurrence scenario of strain fatigue is during the use of polymers, such as polyethylene sheaths for hoisting ropes, shoe soles, rubber bands, artificial skin materials, etc. However, the strain fatigue life prediction models and methods mainly come from Manson Coffin models, SWT models, etc., which were developed by scientists represented by Manson and Coffin through research on dozens of metals or alloys. The property that polymers have viscoelasticity and do not undergo strain fatigue in extremely low strain ranges is considered, a 4-parameter model has been established by introducing strain fatigue threshold (εth) value in Manson Coffin model，and compared the life prediction in AS, PA6, PC, PE-LD, and PE-LLD with Manson-Coffin model. Research has found that the 4-parameter model is suitable for polymers with good flexibility. Compared to the Manson Coffin model, the strain fatigue life of polymers, especially high cycle fatigue life, can be more accurately predicted by the 4-parameter model. And lower plastic deformation does not cause accumulation of polymer fatigue. This work provides a more accurate model for predicting the strain fatigue life of flexible polymers.