In large scale building fire, it is improper to predict smoke descending using traditional simple two-layer zone model, which divides the total space of the building into upper hot smoke layer and lower cool air layer. In this paper, an improved method, named Cell-Zone Method, is used to solve this problem, which first divides the total space into some small subspaces and then uses traditional two-layer zone model in each subspace. Comparison is carried out between these two methods in fire smoke development simulation in typical large space buildings by CFAST4.02 software package. Results show that Cell-Zone Method demonstrates more applicability than traditional simple two-layer zone model in large scale building, especially in buildings having large scale in one direction.