Considering the gear pump performance and the important factors influencing the performance end clearance, the mapping relationship was discussed between performance, clearance and tolerance. The concept of the optimal clearance was introduced, a new function was defined 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 was established, using the above optimization model to calculate the gear pump gear mechanism and the assembling tolerance of ring, it 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.