Background Vertical cavity surface emitting lasers (VCSEL) have very high application value in space radiation environment. Purpose This study aims to explore the degradation rule and mechanism of 850 nm VCSEL in harsh radiation environment. Methods First of all, the MULASSIS tool was employed to calculate displacement damage dose (DDD) and design experimental scheme for 850 nm multimode VCSEL samples. Then, 3 MeV and 10 MeV proton irradiation experiments were conducted to obtain the degradation rule of parameters such as light output power and threshold current with the proton fluence, and to find that the degradation degree of light output power and threshold current were equal under the same DDD. Finally, the Silvaco software was used for modeling and simulation on an experimental basis to extract microscopic parameters such as trap density, donor and acceptor ionization density, mirror loss, radiation recombination rate and photon number. Results The simulation results are in good agreement with the experimental results, these results show that each parameter changes to different degrees with the increase of proton fluence. Conclusions The parameter degradation law and radiation damage mechanism of VCSEL can be deeply explored by simulation on the basis of the experimental law, and simulation results are of great significance for understanding the degradation mechanism of VCSEL.