Due to their high flexibility, ductility, adaptability and safety, soft robots attract wide interests from robotics field as well as related areas. While the large deform able nature of the material and structure of soft robot, rises the great challenge for the control and modeling. In available literatures, research work on soft robot modeling was limited to piecewise constant curvature assumption, which will result the disagreement between the mathematical curve and practical results. In this paper, we proposed the method of continuous modeling via calculus on the active face and restricted face of the developed continuous soft robot. Considering the influence of the robot mass on the motion, the virtual work principle and beam theory are used to establish the quasi-steady state model. Subsequently, the kinematic model between the inflated air pressure and bending angle of the robot was obtained. Experimental results verified the proposed model with average distance error of 10.2%, which is quite for soft robot.