Abstract: In this paper Markov theory is employed to discuss the behavior of bubbles in gas-liquid-solid fluidized bed. The phase holdu pdistribution is thought as a stochastic probability, A simplified stochastic model of gas holdup distribution is developed. The calculation relation of model parameters in the stochastic model are analysed according to the physical mechanism. In order to deeply study the phase holdup distribution of three-phase fluidized bed by the stochastic model, the RTD of gas or solid phases should be given as follows: The results show that the calculation values of εg from the stochastic models were very close to the experimental data.