The power transformer is an essential equipment in transmission and distribution system. Its failurerate is closely related with its working temperature. An increase of temperature by 6℃ will double the aging speed of the transformer and decrease its operating life by half. With the aid of temperature measuring and control device, proper use of the transformer and immediate switch to the cooling device could avoid or decrease faults caused by high temperature of the transformer,thereby, prolong the operating life of the transformer. Specially designed for measuring and controlling oil temperature in the top layer of the transformer, transformer oil surface thermometer of BWYseries possesses favorable protective performance and can be used under outdoor conditions for a long term .Three sets of adjustable temperature switches are installed inside the meter , respectively for start-up of multiple sets of cooling system, over-temperature alarm and so on. Meanwhile, it could transfer the temperature signal to the remote computer system and make it real to synchronously display and control the transformer temperature.