It is always hard to meet the requirement of injection allocation rate of the injector in the Shengli Oilfield due to high viscosity of crude oil and polymer/viscosity reducer solution，and the wells are forced to produce intermittently. There is no specific way to determine the maximum injection speed of the polymer/viscosity reducer combination flooding，which restricts the chemical flooding development in the heavy oil reservoirs. Based on statistics principle，reservoir numerical simulation and other methods，the typical numerical simulation model of the combination flooding was established for the Ng41-51 unit of eastern area in Gudao Oilfield，and the main control factors were screened out by Plackett-Burman experimental design. A prediction model for dimensionless water injectivity index of the polymer/viscosity reducer combination flooding was established finally by multiple regression analysis. On this basis，a method for calculating the injection rate limit of the polymer/viscosity reducer combination flooding was formed. The results show that the calculated value and predicted value of the dimensionless water injectivity index gained by the numerical simulation plan agrees well with that from the verification plan with an average error of 1.71%，which verifies the reliability of the prediction model. When the polymer concentration was 2 000 mg/L，the viscosity reducer concentration was 0.4%，the viscosity reduction rate was 90% and the injected slug size was 0.4 PV，the calculated limited injection rate of the target area was 0.106 4 PV/a. It is very close to the injection data（0.11 PV/a）in the actual situation in the field.