A phosphate solution, free of chromate, fluorides and nitrite, was prepared to form the corrosion resistance conversion film on the AZ31 magnesium alloy surface via chemical deposition, and the effects of film-forming temperature and free acid on surface morphology and corrosion resistance of the film were investigated via CuSO₄ pitting corrosion test, scanning electron microscopy and potentiodynamic polarization curves. The denser film with grain of 20 μm, whose time of CuSO₄ pitting corrosion resistance is longer than 5 min, is obtained on the following process: film-forming temperature 95℃, FA 4~5, ratio of TA/FA 15~20. The E_corr with the film is shifted positively 110 mV(vs. SCE), the I_corr is decreased approximately by three orders compared with that of the bare substrate AZ31 respectively. The results show also that the existence of the film has a great inhibitive action on anodic dissolution and restraint action on the cathodic hydrogen evolution. The dense film is not obtained when the film-forming temperature is lower than 75℃, but increasing with the film-forming temperature, the grain is refiner and the film is denser, which can improve corrosion resistance performance of AZ31 magnesium alloy. Unfortunately, higher film-forming temperature is easy to accelerate hydrolysis of the phosphate and cause plenty of residues phosphide. But the decreasing of residues phosphide can be effectively controlled by controlling the adding amount of the free acid, which further improve quality of the film and decrease the cost.