The thermolytic process of serine is tested by thermogravimetry (TG), differential thermal analysis(DAT) and infrared spectrum (IR) methods. The thermolysis of the serine after the loss of the CO2 is very complicated and it is hard to obtain thermolytic reaction mechanism by experimental testing methods. Thus, the thermolytic reaction mechanism of serine is theoretically deduced by ab initio calculation method of quantum chemistry. The parameters of the total energy and Mulliken population are obtained by the optimized calculation on the geometries of serine, its thermolytic intermediate and final products respectively at the RHF/6-21G level. The calculation of epoxy intermediate is optimized by directly inputting into computer molecular fragments of serine after its loss of NH3. Oscillation analysis is made on the epoxy intermediate and the results indicate that the structure of epoxy intermediate is a stable structure, but not a non-saddle point transition state. The obtained results are consistent with what is stated in the documents concerned. However, in Ratliff′s documents, the production of epoxy intermediate in the thermolytic process of serine is only a supposition rather than based on either experimental test or theoretical deduction. Here the existence of epoxy intermediate is proved theoretically. Through the analysis of the results of the experiment and calculation, the thermolytic reaction mechanism is put forward: the thermolysis of serine is performed with the loss of CO2 as the main reaction channel and accompanied by the secondary reaction with the production of epoxy intermediate after the loss of NH3. Furthermore, a comparison is made between ab initio calculation method and semiempirical method. It′s proved that both methods can result in reasonable conclusion concerning the thermolysis of serine, however, the former has an advantage. In deducing the thermolytic reaction mechanism of serine by semiempirical method, not only is bond order used, but also localized orbital energy is required to judge the strength of bond. When bond order and localized orbital energy as criteria are contradicted with each other, the oscillation of bond is needed as subsidiary judgment. Thus, it is rather inconvenient to deduce the thermolytic reaction mechanism of serine by semiempirical method. On the other hand, by ab initio calculation method, only Mulliken population is needed as criterion to judge the strength of bond and reasonable conclusion can be reached just as well. What′s more, the research by ab initio calculation method can prove the existence of epoxy intermediate, which cannot be tested either by experiment or by semiempirical method.