Abstract: A scale analysis is made of large scale tropical disturbance in a moist atmosphere. dynamic Considering a disturbance of characteristic scale of 1000 km, we obtain a dynamic system in which the vorticity equation has the term of horizonal divergence. This system is not only different from that obtained by Charney in a dry tropical atmosphere but also different from that obtained by Murakami in a conditionally unstable tropical atmosphere. This results is in accordance with observations in tropical atmosphere and confirms professor Xie Yi-bing idea that the atmospheric moisture often gives some very important influence on the atmospheric motion of almost all scales.