In the polishing process of free surface, the traditional intelligent control robot has some problems such as complex programming complexity and difficulty in controlling. A humanoid wrist joint compliant parallel mechanism 3SPS+S used in polishing robot was designed according to the principle of inverse adaptive motion of flexible, and its moving platform can output three rotations. Firstly, the inverse motion equation of the mechanism was established to obtain the relation between deformation quantity of the three flexible branched chains and attitude of the moving platform. Secondly, the mechanics model was deduced through analyzing deformation forces of flexible chains, grinding force and output moment of the tool. Furthermore, simulation was executed based on the 3D model for the target of the tool posture change and the constant grinding force, and the ranges of some work parameters were obtained through the prototype test. Lastly, taking a car shell as a grinding object, an experiment was designed to validate whether the grinding tool mounted on the moving platform could fit rapidly to the shell surface when humanoid wrist joint compliant parallel mechanism wholly moved along a given simple track under driving of a 6-DOF manipulator. Both simulation and test results showed that the position and attitude of the tool can be changed rapidly as changing the geometry of the workpiece through using the grinding mechanism, and it can keep the contact force unchanged between the grinding head and the workpiece. Because of several advantages, such as simple design, flexible movement and convenient control, this kind of mechanism previously has good application value.