In the fully variable hydraulic valve actuation system, the intake valve is opened by a hydraulic valve actuator which is set between the camshaft and the intake valve, and it is closed by a reliever which could relieve the oil pressure in the hydraulic system. By measuring the intake valve lift, it is proved that the hydraulic valve actuation system can realize continuously variable valve lift and the Early Intake Valve Closing (EIVC) by changing Collapsing Angle of the Reliever Oil Mechanism (CAROM). By reasonably matching the structural parameters, the fully variable hydraulic valve system can achieve stable valve opening and seating.Instead of using a throttle, the intake mass flow is controlled by the mode of EIVC. The intake performance of the unthrottled SI engine was studied in this paper. In order to simulate the intake performance, a one-dimension nonsteady flow model was established. It was found that the simulation results of intake pressure were in good agreement with experimental values. The research results showed that the intake pressure at the medium and low load in unthrottled SI engine is larger than that of Throttle controlled (TC) engine and the volumetric efficiency of unthrottled SI engine was improved over all speed range. The pumping losses of unthrottled SI engine were reduced at medium and low load, which lowered the in-cylinder temperature.