Phase function of the compley distributed systems is the significant parameter in the field of radiative heat transfer in medium. The Monte-Carlo method has the advantages of both numerical algorithm and experiment measurement. Multiple scattering phase function of the complex distributed systems based on the Monte-Carlo method overcomes the former deficiency and expresses the influence of the multiple scattering, also, it can calculate phase functions of any controlled volume. According to a number of mathematical experiments, the larger the density of the medium is the better-proportioned the scattered energy is in the 4π solid space.