For transformer protection， the current methods such as power differential principle， recognition of magnetic flux characteristics， identification of equivalent circuit parameters and transformer loop equations all require that the primary and secondary winding leakage inductances and resistances are known in advance. This paper proposes a method to identify three-phase leakage inductances only by measuring the instantaneous no-load closing of the line voltage and the line current， without changing the current transformer configuration on the triangular side. By using the constrained least squares method twice， the primary and secondary winding leakage inductances can be quickly and accurately identified. The EMTP simulation and experimental results of real transformer indicate the algorithm is effectiveness and has good precision.