Among the car accident casualties, Pedestrian occupied a large proportion of the injuries. Pedestrian protection became a key problem of vehicle safety. The engine hood was the most possible zone impacted by the head of pedestrian. The pedestrian protection of engine hood was an important performance of vehicle. The active engine hood was used to improve the pedestrian protection performance of vehicle and to meet the regulations. In the lifting system of active engine hood, springs were used to improve the pedestrian protection performance. A design optimization method about the spring stiffness used in active engine hood was developed in this research. The spring stiffness of K was set as design variable, and its interval was set 1~50 N/mm. The impacting zones of engine hood where HIC values of original engine hood were higher than the mean values of corresponding child and adult zones were selected as the cases of optimization. The objective was to minimize the sum of HIC values of those selected zones. The first type of constraints was that the HIC values of child zones were less than the mean HIC value of child zones. The second type of constraints was that the HIC values of adult zones were less than the mean HIC value of adult zones. The spring stiffness optimization model about active hood based on pedestrian protection was constructed and solved. The results of optimization indicated that the obtained spring stiffness by this method improved the pedestrian protection performance of engine hood.