Abstract: The exact solutions of diffusion and release model of noble gas and halogen fission products in UO₂ particle of HTGR were built under the conditions of adsorption effect and other physical processes. The corresponding release fractions (F(t)) and the ratio of release and productive amounts (R(t)/B(t)) of fission products were also derived. Furthermore, the F(t) and R(t)/B(t) of ¹³¹I, ¹³¹Xe^m, ¹³³Xe and ¹³³Xe^m whose half-lifes are several days in UO₂ particle with the exact solutions, approximate solutions and corresponding numerical solutions under different temperature histories of reactor core were investigated. The results show that the F(t) and R(t)/B(t) are different in numerical values unless the time of release is long enough. The properties of conservation of exact solutions are much more reasonable than the ones of approximate solutions. It is also found that the results of exact solutions approach the actual working conditions more than the approximate and numerical solutions.