Some traces of carbon black particles will be left on the wall surfaces when smoke flows over in a room fire, which are very useful for fire investigators to judge fire point, fire cause, and fire responsibility. According to the particle deposition theory,  a mathematical model for these traces was built and embedded  in the FDS software. The new FDS software with carbon black trace model can not only quantitatively calculate the mass of soot  left on the wall surfaces but also make these simulation values visible. Then a room fire was simulated to verify the validity of the model. Based on  these simulations, the  developing characteristics of the carbon black trace was analyzed.