An experiment of ms laser pulse irradiation on monocrystal silicon was performed. The real-time stress damage from the interaction between ms laser pulse with monocrystal silicon was measured by Mach-Zehnder interferometry. A digital simulation model for ms laser pulse irradiation on monocrystal silicon was established by COMSOL Multiphysics finite element method. The evolution law of a stress field with time under the same pulse width and different energy densities was explored when the monocrystal silicon was irradiated by ms laser pulse from theory and experiments. The processing method for interference fringes was researched and the 45°projection method was proposed to calculate the stress values of materials in different directions based on the traditional x axis projection method. Based on the numerical simulation results, the experimental results of the two methods were compared and analyzed. It shows that the error from the experimental results of the x axis projection method is in 9.5%—29.3% and that from the 45°projection method is in 0.1%—22.6% as compared with that of numerical simulation results. It is verified that the method of 45°projection is more accurate for the processing of interference fringes in the experiment of measuring the real-time stress damage by the Mach-Zehnder. The experimental results provide the theoretical and experimental guidance for the study of the stress damage of monocrystal silicon.