Abstract: The distribution transformer is the main power equipment at the end of the power system. The safe and reliable operation of the distribution transformer directly affects the running condition of the power supply and distribution system and the reliable power supply of the user. The distribution transformer of the traditional pole type installation is protected by the outdoor drop-out fuse, the selection and operation of drop-out fuse have the disadvantages of large protection characteristics, poor overload protection ability, high failure rate of large current, and easy intrusion and winding of foreign objects. The fault has become one of the main factors affecting the reliable operation of the pole type distribution transformer. This paper introduces the principle of built-in high voltage protection distribution transformer, and puts forward a new type of distribution transformer protection configuration with the combination of magnetic holding load switch and the backup current limiting fuse. By comparing with the selection methods and functional characteristics of falling fuse protection curves at home and abroad, it establishs the selection and coordination model of the time current characteristic protection curve of the magnetic holding load switch and the reserve current limiting fuse, and determines the selection and arrangement of the components of the magnetic holding load switch and the backup current limiting fuse. It selects a magnetic holding load switch based on soft magnetic material. The current and oil temperature are combined on the magnetic retaining load switch soft magnetic material. When the switch sensor reaches a certain temperature, the switch has the stability characteristic of the loss release. The magnetic load holding switch structure meets the requirements of three phase linkage mode, and avoids the transformer phase missing operation compared with the traditional drop fuse. When a large current fault occurs in the primary coil of the transformer, the backup current limiting fuse can remove the fault more quickly and avoid greater damage to the transformer. The back-up current limiting fuse is used in combination with the magnetic retaining load switch. It overcomes the problem that the damage of the traditional drop fuse is frequently replaced and the large current fuse is easily broken. It can optimize the overload and short circuit protection performance of the distribution transformer, and can effectively enlarge the protection range of the distribution transformer, the protection features are more rapid and accurate. The development and test of built-in high voltage protection distribution transformer are completed. The functions of Magnetic load switch in low voltage side short circuit protection, overload protection, low level locking function is verified. The operation effect and technical economy of the built-in high voltage protection distribution transformer are analyzed,and it’s popularization and application in the future is prospected.