If particles are mixed with other type of particles, the light scattering characteristics will be changed. In this paper, the light scattering characteristics of water clouds mixed with black carbon are studied. Based on the Mie theory, the single scattering phase function, the single scattering albedo and the asymmetric parameters of water clouds and black carbon aerosols are computed. The Monte Carlo method for simulating the scattering of mixed particles is given, the methods of sampling particle types, sampling free path length, and the sampling scattering directions based on the Mie phase function are given. The changes of the scattering characteristics for water clouds mixed with black carbon aerosols are studied. Reflected light intensity varying with observation angle is calculated with the light normal incidence, and the plane albedo is computed as a function of incidence angle. The influences of mixing ratio and effective radius of black carbon on the scattering characteristic of mixture are discussed. The results show that the black carbon can greatly increase the absorption of the clouds, and the ratio and the size of black aerosols can greatly affect the scattering characteristics of the mixture.