Based on the continuum model for binary solid-liquid phase change system, the mathematic model to simulate the process of laser cladding in situ synthesis TiCp/Al composite on the surface of aluminum alloy was formulated. The additive source method was employed to treat with the heat release of synthetic reaction in powder mixture at certain temperatures, and also to solve the momentum and heat transfer caused by the relative movement between laser beam and the specimen. Two different types of driving forces for flow were considered in the model, i.e., the buoyancy force and the surface tension gradient at the laser pool surface. The three-dimensional transient temperature field simulation program was developed being based upon the commercial software PHOENICS. The calculated and observed fusion boundaries were compared and very good agreement was obtained.