The invention relates to a transformer CT transformation ratio testing device and a testing method. The transformer CT transformation ratio testing device comprises a current receiving host, a secondary mutual inductance current clamp connected with the current receiving host. The transformer CT transformation ratio testing device further comprises a current measuring coil, wherein the current measuring coil comprises a non-ferromagnetic flexible base rod, and the non-ferromagnetic flexible base rod is uniformly wound with an annular coil. The current measuring coil has high flexibility, can be changed in form freely, is applicable to measurement for mutual inductance current of various wires and measurement for mutual inductance current of wires under various environments and can adapt tomutual inductor measurement under any environments. The transformer CT transformation ratio testing device is simple and convenient to operate and high in safety coefficient, and effectively improvesthe working efficiency of measurement. The whole measurement is simple in step and convenient, thereby being suitable for wide popularization and use.