Abstract: In response to the hazard that the bottom emergency oil drain valve of the Mitsubishi main transformer at a hydropower station is blocked and locked, preventing close?range operation during a fire, a retrofit scheme is proposed. It retains the original valve body and installs an extended pipeline with a manual ball valve downstream. Key construction procedures include pipe sizing, valve installation, air?tightness testing, and vacuum oil filling. After the retrofit, oil draining can be performed safely outside the transformer room, reducing the response time from several minutes to within one minute. Six months of operation have verified that the device is reliably sealed and effectively actuated, successfully resolving the problem of unsafe oil discharge in emergencies. This provides a useful reference for similar fire?protection upgrades of aged imported transformers.