Theoretical thermal desorption and in situ temperature programmed desorption(TPD) were developed to study the degassing of contaminants adsorbed in III-nitride photocathodes assembly. In the frame of Malev′sadsorption-diffusion outgassing theory, one dimensional slab was used to model the photocathodes assembly. By using the first and the second Fick′slaw, the expressions of the special gas concentration, the specific gas outgassing rate and the amount of degased specific gas over the time in the degassing process were obtained. To make it more intuitional, these parameters were ploted over time within different diffusion coefficient magnitude at the approximation of fourth order. In the TPD method for III-nitride photocathodes assembly, residual gas mass spectrum was thoroughly studiedat different constant temperature stage to determine which gases were desorbed into the vacuum. The least square method was employed to fit the expression of the specific outgassing rate at the constant temperature of 1 000 K, and the diffusion coefficient D for outgassing N2 was 5×10-5 cm2/s in the dynamic method of degassing process. By combining theoretical analysis with TPD experiments, the heating up temperature for sufficient outgassing the contaminants from the III-nitride photocathodes assembly was evaluated and verified.