According to the topological design theory of parallel mechanism (PM) based on position and orientation characteristic (POC) equations, a three degreesoffreedom (DOF) 3Pa+2RSS PM was designed. The main topological characteristics such as the POC set, DOF and coupling degree(κ) were calculated. It was proved that the coupling degree of the PM equaled to 1(κ=1) by using the formula of coupling degree. Afterward, a forward solution model for the kinematics modeling principle based on ordered single open chain (SOC) units method was established. Then the forward kinematics was solved by using onedimensional search method. The inverse kinematic formulas were derived by establishing the relationship between the input variables and output variables. Meanwhile, the Jacobian matrices were derived by inverse kinematic formulas which were used to analyze the geometric conditions of three kinds of singular positions for the PM. In order to enlarge the workspace of the mechanism, two RSS chains were replaced by using RUU chains under the premise of constant degreesoffreedom, output characteristic and kinematics analysis. The workspace boundary and singularities inside the workspace of the mechanism were analyzed by using the discretization method and calculated in Matlab. The result turned out that the shape of workspace boundary of the mechanism was regular and large. Also the free singularities area inside the workspace was large. The research laid a theoretical foundation for the stiffness, dynamics investigation and prototype design of the PM in the future.