In this study, a multiphase field model for isothermal chemical vapour infiltration (ICVI) process of SiC composites from methyltrichlorosilane (MTS) is developed. The model consists of a set of non-linear partial differential equations by coupling Ginzburg-Landau type phase field equations with convection diffusion type mass balance equations and Navier-Stokes equations for fluid flow. In order to incorporate the thermodynamics of the pyrolysis of MTS into phase field model framework, the reduced chemical reaction mechanism is adopted. The microstructure of preferential deposition of Si, SiC under high ratio of H2 to MTS is simulated and the result is in good agreement with the investigation. The potential risk of blockage of the premature pores during ICVI process is predicted. The competitive growth mechanism of SiC grains is discussed.