Abstract: A stochastic approach, namely, Monte Carlo approach, and "tree" structure have been adopted to simulate random backmixing, unreacted fraction of solid particles including multi-size particles, and dynamics of particles with complex reaction in a complex flow system. Turbulence of particles in the reactor system is simulated by stochastic tree pattern which is shown by pseudorandom number series picked from a uniformly distributed set of numbers between zero and one. A Monte Carlo procedure is employed to model particles stochastic residence time, random flow and reaction. A large number of discrete bundles of particles have been simulated and trailed. The approach has simple physical concept and imitative procedure. All calculations have been carried out easily with a computer. For a simple reaction system, the results are in good agreement with those obtained from other known approaches. The approach could provide a new kind of technique for simulating behaviors of particles in continaous flow reactor.