The residual line birefringence in the elliptic maintaining fiber is easily affected by the environment , so the performance of the Fiber Optics Current Transformer ( FOCT) is easily affected by the environment. By using qua- ternion method , the problem of operating point instability of FOCT due to residual line birefringence is analyzed theo- retically. The results show that the ratio of circular birefringence to residual line birefringence RCL ( circular/line ratio) directly affects the stability of the operating point , and the general trend is that the operating point becomes more stable with RCL increasing. When RCL <2 , the operating point is extremely unstable ; on the other hand , when RCL >4 it be- comes relatively stable. Therefore , the parameter RCL is an important specification to measure the elliptic maintaining fiber , and should be measured before making the FOCT to ensure the performance of FOCT.