Abstract: It is proved in this paper that NWP systematic forecast errors in zonal mean circulation are due to the difference in westerly acceleration process during the forecasting period between the model and real atmospheres.Those forcing factors which evoke the zonal wind variation can be split into various linear terms according to the non-acceleration theorem in a primitive equation system. By applying this scheme to diagnosis of the forecast products of the T42L9 model,it is indicated that the model has the ability of forecast the zonal mean wind to a reasonable existent,but there are still some errors in several places. The results of analysis by employing this scheme show the reason responsible for the systematic forecast errors of the zonal mean wind in the model and the possible way of improving it.And it is also shown that non-acceleration theorem can be used as an efficient tool to diagnose the physical processes of NWP models.