The thrust of a ramjet measured in the direct-connected experiment was obviously different from the thrust based on 1D theoretical calculation, to analysis the cause of above phenomenon, the research on the effect of uniformity of flow field on thrust was conducted on the basis of 3D numerical simulation, the results show that the uniformity of flow field makes a obvious difference between the results by 3D simulation and the results by 1D theoretical calculation on the stagnation parameters in key section of ramjet. Therefore, the thrust acquired by 1D theoretical calculation is incorrect under the condition that flow field is uneven. Furthermore the method to predict the effect of uniformity of flow field on thrust is presented.