The influence of round-off error on summer climate simulation in the northwestern Pacific and East Asia has been investigated by using AGCM(atmospheric general circulation model) which is compiled by single precision and double precision. It is found that double precision computation has better skill than single precision one when simulating zonal wind at 850 hPa in the region. And the reason is also investigated, and the simulating results of 850 hPa zonal wind are mainly studied. The first empirical orthogonal function(EOF) mode of 850 hPa zonal wind simulated by double precision computation is better than that by single precision one, and the second mode time series of the double precision computations has a higher correlation coefficient with the second EOF mode time series of observations. Results show that the round-off error may result in higher atmospheric internal noise in the northwestern Pacific and East Asia, which can weaken the model simulation skill, therefore the double precision computation can better simulate climate than single precision one in this region.