Abstract: The mutual impedance parameters of 500 kV double-circuit and multi-circuit transmission lines are crucial for relay protection settings. The traditional complete outage measurement method results in significant economic losses and lengthy implementation periods. To address this issue, this paper proposes a partial outage measurement technique based on off-frequency signal injection. By taking a single circuit out of service while keeping adjacent lines in operation, and utilizing 45 Hz and 55 Hz off-frequency signal injection combined with GPS-synchronized phasor measurement and adaptive filtering algorithms, accurate extraction of mutual impedance parameters is achieved. Field tests on the 500 kV Taihe-Qianzhou transmission line demonstrate that the zero-sequence mutual impedance measurement error is within ±2.8%, measurement time is reduced by 62.5%, and outage-related losses are reduced by 1.8 million yuan per test, validating the effectiveness and engineering practicality of the proposed method.