The invention relates to a main steam flow online calculation method of a power station based on performance testing data. In a performance testing, a relation curve of tested main steam flow and electrical load based on calculated main steam flow is drawed, and the tested main steam flow is calculated based on condensed water flow which is measured through ASME (American Society Mechanical Engineers) flow nozzles; in an actual operation, corresponding testing main steam flow on the curve through the electrical load is found out and main operation parameters are corrected to obtain the main steam flow in the actual operation. The calculated main steam flow through the online calculation method is small in deviation and can be consulted and performed economical analysis by power station.