A composite film containing heteropolyanion was fabricated by attaching the Keggin-type phosphomolybdic PMo 12O 40 3− on l-cysteine self-assembled monolayers film modified gold electrode by electrostatic adsorption. ATR-FTIR, SEM, AC impedance and cyclic voltammetry, chronocoulometry were used for the characterization and electrochemical properties of the composite film. Two reversible redox couples were observed in 1.0 mol l −1 H 2SO 4 in the potential range of 0.2–0.8 V, which was attributed to two-electron and two-proton electrochemical processes of PMo 12O 40 3−. Charge transfer coefficient is 3.76×10 −8 cm 2 s −1. FTIR studies indicated that there was some electrostatic interaction between PMo 12O 40 3− and surface NH 3 +. Compared with PMo 12O 40 3− in the solution, the composite film electrode can exist in a larger pH range and has a good sensitive respond to H +, which can be used as pH-potential sensor. Moreover, the composite film shows good catalytic activities for the reduction of bromate anion (BrO 3 −) and nitrite (NO 2 −) in acidic solution. The catalytic mechanisms were described.