This paper presents a physically based simulation algorithm for animating viscous fluid. This algorithm introduces an equivalent energy model to couple diffusion processes and projection processes into a single linear system for solving together. This model implicitly solves the viscous term while simultaneously solving pressure to guarantee incompressibility of fluid. Furthermore it automatically captures the vital zero-traction boundary conditions, eliminating artifacts caused by directly approximating this boundary condition. Furthermore, this paper utilizes the physical information taken by particles to solve the advection term for battling numerical dissipation and constructs an implicit surface of fluid based on particles. Finally, the test results show the efficiency, accuracy and stability of the algorithm, and it can nicely simulate deformation characteristics of various viscous fluids, efficiently supporting variable viscosity.