Rate equations describing the dynamic processes in Er3+ doped pentaphosphate crystals were built up. Using this rate equations the dynamic processes of the building up processes of the populations of the states of Er3+ ions in ErP5O14 crystal was gotten. The spontaneous radiative processes, multiphonon nonradiative processes, absorption of the pump photons and the possible energy transfer processes between all states of Er3+ ions were included in the rate equations, and the pump power was supposed to be unchanged with time. Considered that the equation n(t0+(Delta) t)=n(t0+dndivided bydt(Delta) t would be held if (Delta) t was small enough, and using the initial condition n1=1.0, ni=0(idoes not equal1), the time dependent populations of all states could be gotten from the rate equations. When the pump power was 103W, population of the ground state decreased rapidly with time and then tended to stable, at the same time, the population of 4F9/2, the state excited directly by the pump light, formed a peak-like structure and then tended to stable. The gradient of the decay side and the rising side of this structure was close, which believed to be the result of the large number of the energy transfer processes between 4F9/2 and other states and the great rate of these processes. Populations of the other excited states lower than 4F9/2 were all increase at first and then decrease with time while excited states higher than 4F9/2 were all increase with time and then tended to be stable. The system came to be stable at about 140 microsecond(s) .