Abstract: Dynamics of Populus davidiana forest in Lesser Xingan Mountain, Northeast China, was studied by means of forest dynamics "gap model". After the initial stand conditions were given, changes of stand factors including number of trees, base area, biomass, leaf area index and production during 150 years were simulated. The rule of –3/2 power and law of constant yield of stand density effects were applied to test simulated stand density. The results show that change of the simulated density was consistent with that of theoretical density. This not only tested the forest dynamics model but also verified the theory of stand density effect.