The invention provides an online calibration device for a large-scale test equipment multi-axis force sensor, which can online demarcate and calibrate the multi-axis force sensor in the field without removing the multi-axis force sensor. The calibration device mainly comprises a longitudinal mounting structure, a lateral mounting structure and a force transferring ball body structure, wherein both the longitudinal mounting structure and the lateral mounting structure use screw holes of the sensor base itself for mounting and fixing, both the longitudinal mounting structure and the lateral mounting structure serve as carriers for mounting a standard force source loading device, and the loaded standard force source needs to be ensured to be stable and accurate in direction; and in order to ensure the standard force source applied to the sensor to be more accurate, the force transferring ball body structure is adopted to transfer the standard force source outputted by a force loading device to the sensor, thereby effectively avoiding a force coupling problem when the sensor bears the force unidirectionally. Through comparing the data acquired by the multi-axis force sensor system and the data acquired by the standard force source system, online calibration on the sensor can be realized.