This paper presents a numerical model based on the mass flow rate of seal leakage. This numerical model is considered as a correct method for 3-D numerical simulation. It can be used to simulate the effect of seal leakage at the stator root of a multistage axial compressor. Implementation of the correct method is using a numerical model based on the flux conservation which can control the mass flow rate of seal leakage accurately at the seal cavity of compressor. The mass flow rate of seal leakage is chosen as the key research parameter on the aerodynamic performance effect of the seal engineering application in a multistage axial compressor. Combined with the 3-D numerical simulation methods, an engineering numerical approach is set up in this study. A nine-stage axial compressor is taken as the research object in this paper and its aerodynamic performance is tested for proving the applicability of the numerical model for seal leakage. In the cases of several operating rotation speeds, numerical results of the nine-stage axial compressor performance characteristic curves are in good agreement with the experimental data. It is considered that the numerical approach based on the simplified numerical model in this paper can predict the performance of multistage axial compressor accurately. Then, comparisons are made against different cases of seal leakage mass flow rate for analyzing the impact of seal leakage increasing on the aerodynamic performance of the nine-stage axial compressor. The main point of comparisons is focused on the changes of the overall performance and the flow distribution in the compressor with the seal leakage changing. The results indicate that performance of multistage axial compressor is degenerated faster and faster with seal leakage increasing in all operating working points. An overall decline is appeared in the flow capacity, working capacity, efficiency and surge margin of the compressor. For the impact investigation on the changes of flow distribution, the total pressure loss coefficient, the relative Mach number contours and the movement of streamlines are studied in different seal leakage cases under several operating working points. The result also shows that stators located in front stages of multistage axial compressor are affected more seriously with the increasing mass flow rate of seal leakage. Under the influence of seal leakage, degradation of flow condition in stators located in front stages is more severely than that in back stages, the total pressure loss coefficient and entropy are increased, and the flow separations at the root of stators in front stages are developed faster with seal leakage increasing. So it can be confirmed that relatively larger flow losses in front stages bring significant impact on the decay of aerodynamic performance for a multistage axial compressor.