With the rapid development of smart grid, Wireless Sensor Network (WSN) and Radio Frequency Identification (RFID) have been reported to realize real-time online monitoring. However, WSN requires higher cost and power consumption with respect to RFID and RFID shows lower cost and power consumption while the communication distance is shorter than WSN. Besides, both of the WSN spot and active RFID sensor tag need battery for power, passive RFID sensor tag can work without battery but has limited communication distance and cannot realize real-time online monitoring. Based on the problems above, we design a vibration energy harvester and wireless power transmitter as wireless power supply for passive RFID sensor tag. The vibration energy harvester based on electromagnetic induction is specially designed to be applied on transformer and then provide wireless energy with the radio frequency waves to the nearby tags. In this way the passive RFID sensor tags can operate as the active tags. So, it can realize real-time online monitoring and reduce the layout and maintenance cost at the same time, meeting the requirements of smart grid. The whole device is built and tested on a 10 kV transformer. The measurement results show that the proposed device can efficiently power the passive RFID sensor tag at the distance of 6 m.