A electroless Ni-P plating was obtained on brass surface at medium temperature(70 ℃) in acidic nickel plating system by using nickel sulfate as main salt, sodium hypophosphite as reducing agent, lactic acid and glacial acetic acid as composite complexing agent and ammonium sulphate as accelerating agent. The effects of ratio of main salt and reducing agent,amount of complexing agent and accelerating agent on electroless plating were investigated. The optimal formula of bath was determined. The results show that the ratio of nickel sulfate and sodium hypophosphite significantly influence on deposition rate and hardness. In the composite complexing agent,actic acid mainly plays the role of complexing agent, where as glacial acetic acid mainly plays the role of buffer. Hypophosphite activity is enhanced by ammonium sulfate,which increases the deposition rate.