Abstract: An AC voltage step experiment instrument for mine-used power supply testing was introduced. The instrument uses touch screen as window of command input and voltage indicator, Modbus-RTU as communication protocol, and single chip microcomputer to control AC solid-state relay to realize step among different voltages. Load test result shows that under 127 V voltage level, the instrument can do step voltage experiment safely and reliably, with the max load current of more than 5 A and step time of less than 50 ms.