With the development of airborne distributed units, the requirements of low voltage switching power supply system in distributed units are becoming higher and higher. The switching power supply system with multi-output, comprehensive protection function and high reliability has become the choice of more and more users. However, it is difficult to design the distributed unit low-voltage switching power supply with many output ways and comprehensive protection functions. This paper explores the design of a distributed unit low-voltage switching power supply, designs the related protection function modules, and realizes the needs of multi-output at the same time, and provides a new design scheme for practical engineering practice.