Objective To analyze and assess the postoperative motor function in children with spastic cerebral palsy (SCP) by surface electromyography (sEMG) and joint angle. Methods Sixteen children with SCP were involved in this study. The sEMG of rectus femoris, biceps femoris, semitendinosus, tibialis anterior, lateral gastrocnemius and medial gastrocnemius muscles and joint angles of the hip, knee and ankle during straight walking were collected preoperatively and postoperatively. In every gait phase, the mean values of joint angles, root mean square and integrated electromyography of sEMG were calculated, to evaluate muscle strength and muscular tension quantitatively. Results The muscle tension of lower limbs was significantly decreased (P<0.05). The muscle strength of rectus femoris and biceps femoris was decreased in the swing phase. At the midswing and terminal swing phase, the strength of tibialis anterior increased significantly (P<0.05). The flexion angle of hip and knee decreased significantly (P<0.05). The dorsiflexion angle of ankle increased significantly (P<0.05), and the varus angle decreased significantly (P<0.05). Conclusions After operation, the crouching gait and clubfoot were improved positively. Therefore, the motor function of children was improved. Combining sEMG and joint angle can evaluate the muscle function of patients quantitatively, and it also can provide references for clinical diagnosis.