Through establishment of a two-dimensional cellular automata model, the evolution process of delta accumulation in inland rivers after a mountain river flows directly into open lake areas was examined. The development process of delta accumulation and the evolution laws of alluvial channel morphology on the accumulation surface were analyzed. The whole pattern was determined: first longitudinal forward, then vertical deposition uplift, and finally lateral spreading. The longitudinal forward rate decreases with the increase of the simulation steps, a gradual deposition uplift occurs in the vertical direction, and the rate of lateral spreading decreases with the increase of the simulation steps. The channel morphology evolution on the accumulation surface can be divided into three stages: the straight channel, braided channel, and micro-bend channel. The simulated results agree with existing experimental results of the physical model, indicating that the cellular automata model can be used to simulate the evolution process of the delta.