Using the rate equations which describing the dynamic processes in Er3+ doped pentaphosphate crystals, the upconversion mechanism in this system under 650nm excitation was gotten, and the influence of the concentration of Er3+ ions on the dominant populate process of the upper state of the upconversion luminescence was analyzed. The spontaneous radiate processes, multiphonon nonradiative processes, stimulated transitions 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. It came to the result that under weak excitations, the main upconversion mechanism in ErP5O14 crystal was energy transfer upconversion, and contribution from excited state absorption was small. And some nonresonant energy transfer processes played an important role in building up of the population of the upper state of some upconversion luminescence. Such as nonresonant energy transfer process (3->1, 5->12), the energy mismatch of which is 1585cm-1;31.1% of the ions of 4G11/2, the upper state of 379nm upconversion luminescence, was populated through this processes. Since changed concentration of Er3+ ions would cause changed distance thus changed energy transfer rate between them, and then the contribution of all processes in the system would be different. The result showed in ErxLa1-xP5O14 crystal with different x, the upper states of the upconversion luminescence were populated from different process, with the concentration of ER3+ ions increased, the influence of energy transfer upconversion processes (Including some nonresonant energy transfer) became larger.