Objective To observe the effects of quadriceps exercises with electrical stimulation on the patients who developed extensor lag after knee operation. Method 30 cases after knee operation were randomly divided into 2 groups: quadriceps exercises group A(12cases), exercises with Electrical Stimulation group B(18cases), and both groups were also given routine rehabilitation treatment. Evaluation was given to the lag degree of knee before and after 2 weeks treatment. Result The lag degree of group A was 14.5°±7.33° before treatment and 8.4°±6.75° after treatment, while that of group B was 13.9°±9.15° before and 3.9°±3.56° after treatment. To compare the lag degree before and after treatment, the significant improvement could be found in both groups, (P<0.05) and the improvement in group B is much better than that in group A, P<0.05. Conclusions The effect of exercises with Electrical Stimulation is better than that with pure quadriceps exercises to treat the extensor lag.