In order to overcome the defects of traditional digital millivoltmeter, a kind of new digital display millivoltmeter based on STC89C52 is designed. The millivoltmeter consists of many integrated devices to realize signal transformation. It has many advantages, such as simplified the complexity of system design, enhanced anti-interference ability and accuracy, improved effectively the nonlinear and frequency of the system. It has the functions of automatic range change, overrun protection and alarm. This high-precision broadband millivoltmeter based on STC89C52 can meet the measurement needs for weak AC signal within 1000mV.