In the in-situ calibration of rotor balance, the accurate loading of calibration force is an important factor to improve the calibration accuracy. The structural deformation of the calibration device will cause errors in the loading of the calibration force and affect the calibration accuracy. In order to solve this problem, a force calibration system for rotor balance is designed. The system uses laser triangulation method to measure the micro-displacement, and calculates the displacement of force loading point by measuring the change of spatial position of cubic target. A mathematical model is established to analyze the influence of structural deformation on the loading of calibration force and to measure the calibration force. Experiments show that the system can eliminate the influence of structural deformation, measure the calibration force accurately and improve the loading accuracy of the calibration force.