The invention discloses an automatic test method for flexible direct current leg differential protection. Through manually introducing the flexible direct current leg differential protection logic fixed value, short circuit failure calculating is automatically performed according to the protection fixed value and the protection action equation, after the calculating is finished the size of the measured electric current value required for the differential protection calculating is reasonably configured, and the differential protection reliable action condition can be satisfied by the electric current value. The electric current signal satisfying the differential protection action is applied to the flexible direct current protection by a differential protection test module, whether the differential protection action is the reliable action or not is judged through obtaining the protection outlet information of the flexible direct current protection, and the protection outlet time |T2-T1|can be automatically calculated through comparing a time T1 when the differential protection failure electric current is applied and a time T2 when the protection outlet is obtained. The manual linksof saturation logic analysis and fixed value calculating can be reduced, and the test efficiency is greatly improved.