Abstract: Maintaining a consistent power supply for downhole drilling devices has proved to be very problematic. For that reason, a downhole automatic power supply device was designed. In the case of the MWD instrument powered by disposable high-temperature lithium battery, the problem of seamless switching of multiple power sources in power supply could only be addressed by selecting an appropriate power multiplexing method for a series-parallel battery pack, thus the downhole automatic power supply device was designed. After analyzing the shortcomings of the original power supply method, it was found that the battery utilization rate of the diode power multiplexing method was low. Further, the microcontroller-based power supply technology failed to cope with the instantaneous voltage loss of battery and occupied the microcontroller resources. Through comprehensive comparison, the priority power supply multiplexer chip TPS2121 was used to realize the seamless switching between multiple power sources, and the DC/DC converter was designed by using the LM5166 synchronous step-down converter to form an automatic downhole power supply device. The indoor test and field test demonstrated that this downhole automatic power supply device could achieve the automatic switching of multiple battery packs, improved the battery utilization rate by using each battery pack in sequence, and achieved the seamless power supply of MWD instrument by cooperating with the DC/DC converter. In addition, this device presented high reliability and economic benefits.