As an electronic equipment, the merging unit will be produce noise in the process of A/D sampling, the noise superposition to sampling value will be obvious influence the accuracy of electric energy metering, it is necessary to do noise testing for digital watthour meter before running, and ensure the accuracy of the digital watthour meter.Therefore, this paper studies a kind of metering performance testing system of digital watthour meter with noise testing function, proposed input noise test items and test method on the basis of existing digital watthour meter test items, through the study found that the error of digital watthour meter will be increased and unstable obviously under the condition of noise, and when SNR is 30 dB, the error exceeding it’s tolerance, suggest to increase the input noise testing before the digital watthour meter put into operation.