Considering the gear pump performance and the important factors influencing the performance end clearance, discusses the mapping relationship between performance, clearance and tolerance. Introducing the concept of the optimal clearance, defining a new function for the assembly and the difference between the optimal clearance, clearance with new function and gear pump performance as objective function, the tolerance of key parts machining accuracy as constraint, tolerance optimization model is established, using the above optimization model to calculate the gear pump gear mechanism and the assembling tolerance of ring, provides the basis for the gear pump tolerance optimization design, finally, a type of gear pump as an example, verify the validity of the model.