Abstract: On load tap changer is different from off load tap changer. It can adjust the tap changer gear when the transformer is running. Thereby changing the transformer output voltage. In view of the damage of on load tap changer mechanism box affected by rain, a set of outdoor transformer on load tap changer mechanism box device is proposed. After the device is applied, it can be firmly fixed at the connecting part of the transmission shaft. It is not loose and does not affect the gear adjustment function during the gear adjustment of the main transformer. Realize no water accumulation in the transmission shaft connecting part area. It is applicable to all kinds of outdoor on load tap changer mechanism boxes. It has good anti ponding effect, low cost, simple principle, convenient installation and is suitable for popularization.