It is a challenging problem to efficiently calculate and systematically analyze the motion laws and working gait of the inchworm-like soft robot. A simple mechanical model consisting of a rigid slider and a curved beam is established under quasi-static conditions, in order to realize quasi-static modeling and simulation analysis of the inchworm-like soft robot. First, based on the Euler-Bernoulli beam theory, the total potential energy expression of the beam is obtained. Next, combining the boundary conditions and the governing equation derived from the total potential energy based on the variational principle, a set of ordinary differential equations are established. Then, through discretization and dimensionlessness of those equations, a class of nonlinear algebraic equations for numerical solution is proposed. Finally, in the light of the contact situation between curved beam and ground as well as the viscous and slip condition of the system, the motion of the robot is divided into three stages. Through numerical calculations, the different configurations of the curved beam in different stages with the change of the initial curvature amplitude are obtained, which makes it possible to describe the law, the gait, and the net displacement of the soft robot in a motion cycle and solve the problem of movement connection of soft robots at different stages. The quasi-static method is characterized by high computational efficiency, which is more suitable for analyzing the motion configuration of soft robots.