The work aims to obtain high performance film by studying new process of silicon manganese molybdenum conversion coating. Single factor experiment was performed to determine process conditions. Film-forming process was studied in electrochemical method. Corrosion resistance of the film was tested be performing salt spray test. Adhesion, film thickness and surface morphology of the film was measured by performing cross cut test, using thickness gauge and SEM. Optimal process conditions were selected as: manganese dihydrogen phosphate of 2.0 g/L, sodium molybdate of 4 g/L, tannin of 0.4 g/L, sodium fluoride of 1.6 g/L, fluosilicate of 3 mL/L, pH of 5 and film-forming time of 8 min. The as-prepared conversion coating was amorphous and uniform, average thickness was 2.0 microns, adhesion to the substrate reached grade 0. No sodium nitrite accelerator is used in the preparation process of silicon manganese molybdenum conversion coating. Thickness, corrosion resistance and adhesion of the as-prepared conversion film meet production needs.