Accurate air-fuel ratio control is the key element for fuel economy low emissions and good driveability of spark ingnition engine. In order to more accurately identify the fuel film model parameter and achieve precise control of air-fuel ratio, the film identification equation based on fuel film dynamic model was built under the consideration of model error and random error. According to recursive least squares, the parameter estimation with weighted matrix expression was got, then the weighted matrix was solved using Markov estimation and the estimated value of optimal film parameter was got consequently. An engine model was set up using GT-power, and air fuel ratio, mass injection flow engine speed and air mass flow were obtained by using sampling experiment. Parameter identification applying the proposed algorithm and the least squares were conducted at 1000~3500r/min with throttle opened from 5%~75%, respectively. Meanwhile, the identification results were compared with the calibration value. The results showed that the deposition coefficient X of fuel reduced and evaporation time constant τ of film quality increased with increase of throttle opening degree when speed was the same, and X, τ decreased as speed increasing when throttle opening degree was the same. Compared with the least squares algorithm, more accurate identification result can be got by using the proposed algorithm.