The invention discloses an online monitoring method of the pump assembly efficiency of constant-speed circulating water pumps of a thermal power generating unit. The method comprises the specific steps of: judging if respective circulating water systems of two constant-speed circulating water pumps are located in stable operation working conditions in real time after respective pump characteristicmodels and pipeline characteristic models of the two constant-speed circulating water pumps and the power generating power of the thermal power generating unit are obtained; and calculating respective present lifts and circulating water flows of the two circulating water pumps according to the pump characteristic models and the pipeline characteristic models when the circulating water systems arelocated in the stable state. The present pump assembly efficiency of the constant-speed circulating water pumps is calculated according to the motor power, the present pump lifts and the present circulating water flows of the two constant-speed circulating water pumps. The method builds real-time judging mechanisms of the operation working conditions of the two water pumps according to the pump assembly conditions of the two constant-speed circulating water pumps, and solves the problem of weaker accuracy in calculation of the pump assembly efficiency under unstable conditions due to incapability of distinguishing stable and instable system operation states in the monitoring process in the prior art.