Second harmonic generation (SHG) and d ispersion properties of GaSe crystal are researched by a polarized and tunable, acoustic-optically Q switched CO_2laser.With 250Hz r e peated frequency and collinearly oo-e phase matching type,the experiment realizes the second harmonic generation in GaSe crystal and measures the external phase matching angle θ_m in four fundamental wavelengths:9.3,9.6,0.3and 10.6μm.According to the comparison between θ_m measured and calculated results based on a vailable GaSe dispersion equations,the most suitable dispersion equation for this GaSe sample in the experiment is determined.Besid es,the relationship between SHG output and fundamental frequency azimuth angle φ is mea sured.So this experiment can be a way to determine the GaSe optimum dispersion equations,and be ground work for GaSe sample′s furth er research.