RMSAverage Detector is a new CISPR weighting detector to better consider the Impact of impulsive disturbances used in digital modulation system. The first, this paper put forward difference equation of digital RMSAverage Detector, according to Frequency Response of RMSAverage Detector referencing to CISPR 1611 basic standard. The second, Implementation structure of Multichannel RMSAverage Detector on FPGA is present in this paper. This Implementation structure could directly applicable to Timedomain EMI test receiver based on FFT. Lastly, step response of detector is obtained by simulation based on FPGA. This Method and implementation structure of FPGA is used in Timedomain EMI test receiver, and the application effect is all right.