The development of microcomputer protection device for the low-side of box-type step-up transformer in wind farm is introduced. The overall structure and the operating principle are discussed and the schematic of signal modulated circuit, switch signal input/output, and communication interface are given. This paper analyzes the operating principle of specified time over-current protection, inverse time over-current protection and low voltage protection, describes the software design methods and provides test data of the protection device. The protection device has the functions of collecting parameters of three-phase voltage, three-phase current, active power, reactive power, and power factor, non-electricity protection, remote control, and communication. It can realize such tasks as measuring, protecting and controlling for the low-side of box-type step-up transformer in wind farm.