The article deals with the peculiarities of low-pressure gas at pressures above 102 Pa pumping process with cryoadsorption vacuum pump. Limiting factor of the process is not the rate of molecular diffusion in the adsorbent pores of the gas being pumped, but its warming due to sorption heat release. Low thermal conductivity of the adsorbent granules layer reduces the intensity of heat removal to cryopanels. Changing the temperature distribution in the adsorbent bed is offered to calculate by the finite difference method on the basis of time iteration. Layer is considered as unbounded plate being thermostated by cryopanels at one side hand and having internal distributed heat sources. It is shown that having determined temperature gradient in sorbent layer one can predict the rate of cryoadsorption vacuum pump performance drop. The effect of time-varying local temperatures on the elementary layers pumping speed and, in effect, on the heat release rate in them is taken into account.