The invention discloses a cold end energy conservation diagnosis method for a condensing steam turbine. The cold end energy conservation diagnosis method for the condensing steam turbine comprises the following steps that the exhaust steam temperature (or the back pressure of the turbine), existing before cold end factors change, of the condensing steam turbine is measured; the circulating water inlet and outlet temperatures existing before the cold factors change are measured; the cold end factors which are possible to change are determined, and the parameters, existing before changing and after changing, of each cold end factor are determined; and the exhaust steam temperature existing after the cold end factors change, of the condensing steam turbine is calculated according to the parameters, and the decrease amplitude, generated after the cold factors change, of the back pressure of the condensing steam turbine is obtained. The cold end energy conservation diagnosis method for the condensing steam turbine has the advantages that calculation is easy, the number of needed parameters is small, the precision is high, and implementation is easy; and the defect that during existing cold end energy conservation diagnosis, all the cold end variables are needed is overcome, and real-time high-precision cold end energy conservation diagnosis is achieved.