This paper proposes a new on-line measuring method for tanδof insulation based on passive current sensors.The new method avoids the difficulties encountered during producing sensors with high precision for ratio error and phase displacement error，thus it is cost-effective and of high accuracy.Combining a phase difference algorithm with the computer technology，the method is suitable for micro-controller unit（MCU）.The practicability of the method is validated by field tests，thus its anti-interference ability is confirmed.