Considered with the vertical micro-cavity effects of vertical-cavity semiconductor optical amplifiers , gain properties are analyzed in reflection mode. Based on solving laser single-mode rate equation numerically , the facts are given that optical gain and output saturation power are changing due to the variation of optical pump power or output mirror reflectivity. Dymanic power extension is analyzed simultaneously. A conclusion is made that the increase of output mirror reflectivity is with the increase of optical gain , and strong pump light power will lead to gain flatness and larger gain. Some results are summarized to providing references of practical applications.