A theoretical model was developed to calculate small signal gain, and was used to investigate the influence of focusing condition on the small signal gain for longitudinal pumped Cr3+:LiSAF6 laser. We optimized the small signal gain of Cr3+:LiSAF6 crystal, obtained optimum values of waist radii and focus positioons under given pumping powers. The calculated results proved that the optimization greatly improved the small signal gain perforonance of Cr3+:LiSAF6 crystal.