Electric power monitoring instrument is an advanced intelligent and digital front-end capture devices which has been widely used in various control systems, it replaces the conventional power measuring instruments and transmitters in function. In this paper, power monitoring instrument with harmonic monitoring functions as a design object, focused on using AD421 to realize 4~20mA two-wire transmitter. System use 32-bit freescale chip MCF51AC256B as the CPU. For security reasons, communication interface between AD421 and CPU use 6N136 to execute optocoupler isolation, this paper presents the design of 4~20mA current loop and meter system hardware block diagram. Experiment verify the given current in current loop can be exported stably and accurately, loop design is feasible and reliable.