Dual-robot cooperation has higher stiffness than single robot, but the deformation problem still needs to be studied in its milling application. Aiming at the issues of the existing deformation model based on stiffness model for dual-robot cooperative milling system ignores the deformation caused by gravity and fails to measure the end of milling cutter, this paper establishes a comprehensive deformation model for the end of milling cutter considering joint elasticity, spindle weight, and linkage gravity, on the basis of the virtual joint method and the principle of virtual concentrated inertial and mass stick. The simulation results indicate that the deformation caused by gravity cannot be neglected in milling and validate that the proposed deformation model has high accuracy and effectiveness under diverse postures and loads.