An instrument for measuring the frequency of low-voltage electric network is introduced and its basic principle, software and hardware structure are also described, which is base on MCU AT89S52. The instrument can monitor frequency signal simultaneously and possess such functions as the data storage, communicate with PC server, human-machine communication. The validity of this method is verified by PROTEUS, which is a co-simulation of microprocessor software within a mixed mode SPICE simulator. The results prove that this method has some practical value.