Experimental methods of laser irradiation effect in simulation space condition are introduced. Experimental platforms are built and laser-induced damage experiments are performed to silica glass by YAG laser with nanosecond pulse high peak power. Laser-induced damage thresholds of silica glass in low and high vacuum condition are measured respectively. The morphology and chemical compositions of micro-region in damage speckle are compared and analyzed. The function of laser pulse energy deposition and the express of shock wave expansion pressure are given. Corresponding damage morphology and reasons of threshold change are explained. Experimental results are in accordance with theoretical analysis.