By using 'history reconstruction method', this paper studied the dynamics of two Polytrichum juniperinum populations with and without sporophytes. The population with sporophyte production experienced a dynamic process from increase to stabilization, while that without sporophyte production experienced a process from increase to decrease. Before 2003, both the increase rate and the birth rate of two populations decreased continuously, with the death rate showing the tendency from decrease to a continuous increase. The advantages of 'history reconstruction method' in studying population dynamics were proved and discussed.