This study proposes analysis of mechanism stiffness for a 3-axis AC servo motor parallel motion platform with 3-DOF Z 、α、β. To simplify the complexity of actuator of mechanism, the 3-actuators are designed to lay down the base platform and the actuators that guide the sliders of the prismatic joints are parallel to each other. Inverse kinematics and work space of this mechanism will be calculated and the stiffness of mechanism that offers a designer to consult for optimal structure design will be analyzed by using virtual work principle in this study.