Abstract: Gas-separating efficiency is one of important index to evaluates downhole gas/oil separator,But there are many factors that effect on gas-separating efficiency,such as viscosity,diameter of separator and gas/liquid ration were all Analyzed.According to principle of downhole gas/oil separator,a process of experiment was established to study effect factors of separator efficiency.The test mainly analyzed velocity and gas oil ratio,shows that one of gas oil ratio or velocity was increased,the efficiency of gas-separating more high,the experiment also demonstrated the results of theoretical analysis.