The opening process of the circle flat parachute with the differential size and the differential vent position of the canopy by the steady flow wind tunnel were dynamic tested. The transient opening loads and the inflation shape of canopy were measured and the dynamic relation of them was gained. The results show that the transient opening loads are influenced not only by the aerodynamic forces of parachute, and the force produced by the change rate of added mass, but also by the instability and wave of canopy (or parachute suspension line). At mean time, the mean payload of canopy is nearly equal to 60% of the opening shock for the steady canopy. Besides, the load fluctuation is mostly produced by the "breath" phenomena. The bigger actual vent area of canopy, the better stability of a parachute, but the parachute inflation performance will worsen. The experiment investigation may be helpful for the further research.