There are many fire accidents occurred in Taiwan,which often caused huge damage of life or property ,so the research in the building fire is really important. This paper utilizes FDS field model simulation program to simulate the fire accident occurred in San-Chung city.This fire is a typical fire of stack effect. Computational method has been used to study the upper temperature distribution, gas layer height and concentration of CO in the fire site at the time of fire. And the result offers the information to prevent the fire happen.