The invention provides a main device investment quantity measuring and calculating assessment method and system through electric power metering. The method comprises the steps of (1) establishing various main device measuring and calculating index systems through electric power metering, wherein the index system comprises 1) a newly-added main device, 2) a reconstructing and replacing main device,3) a fault replacing main device; (2) based on the measuring and calculating index system, and by means of historical data and expert experience method, the reasonable investment quantity required byvarious electric power metering main devices for next year can be calculated, and the investment quantity reported by each company can be assessed, and project decision and implementation can be guided. The system comprises a database module, an input module, an analyzing and measuring and calculating module and an assessment output module, wherein the database module stores the historical quantity data and business data related to the new adding, reconstructing and replacing, and fault replacing of various main devices in each company; the input module obtains the historical quantity data and business data related to the new adding, reconstructing and replacing, and fault replacing of various main devices in some company, which require to be measured and calculated, from the database module; the analyzing and measuring and calculating module works based on the data input by the input module; the assessment output module conducts reasonable assessment on the new adding, reconstructingand replacing, and fault replacing of various main devices in some company based on the data measured and calculated by the analyzing and measuring and calculating module.