Taking robot arm as research objective, this paper analyzes and researches the stiffness model of a machining robot. First, the size of the equivalent stiffness is calculated according to the robot transmission components, which are come down to elastic components and reduced to the corresponding joints. After obtaining the stiffness of each robot joint, the joint deflection performance in robot end is mapped out by using Jacobian matrix. In addition, the robot arm is regarded as a flexible rod, and the deflection deformation models of stretch, torsion and curve are built, respectively. And then the robot kinematics model is established, in which the staging parameters are contained. From calculating the operation end mapping, each arm rod deflection has micro variable. Finally, the joint deflection and arm rod deflection are stacked in the robot operation end, and then the robot general operating space stiffness is solved through stiffness definition.