The flow degradation of gear pump was induced by the contaminant particles entrained in the fluid within the power shift steering transmission (PSST). On the basis of the degradation mechanisms of gear pumps, the mathematic model for the broken process of contaminants was proposed through the investigation of contaminants entrained into the leakage path in the clearance between the gear ends and the end plates. Within the model, both the values for the particle broken constant and the leakage rate were determined. Thereafter, the degradation model for gear pump was derived for the inference on the relationship of the contaminant mass with wear parameter. In addition, the effects of the contaminant level and the construction parameters were also analyzed. Results showed that the contaminant wear, including the process of particle broken and wear debris generation, can be well interpreted by the degradation model. Meanwhile, the prediction method of contaminant sensitivity for gear pumps was provided by the model, which was valuable and practical for the improvement of hydraulic and lubricated system in PSST.