In order to realize the highest transmission efficiency in a Microwave Power Transmission (MWPT) system, microwave transmitting beam should be directed to the receiving end. Through a closed-loop regulation and control of phases of the microwave transmitting antenna unit, tracking and locking of the maximum receiving power is realized. In this paper, a microwave power transmission system with two units of X-band transmitting antennas is built and adaptively adjusting the phases at the transmitting end to achieve the highest efficiency is realized based on LabWindows/CVI platform. The center distance between the transmitting antenna and receiving antenna is 152 cm and the minimum phase shift step of power amplifier is 5.625°. Measured results show that the 3 dB dynamic range of the output power of the microwave wireless energy transmission system has been increased by 49%, and it controls the phase accurately and locks the highest system efficiency with an adaptive closed-loop.