The invention discloses a method for evaluating performances of a condensing steam turbine after high back pressure improvement. The method comprises the following steps: distributing sufficient pressure, temperature, flow and electric power test points on a thermodynamic system of a turbine unit; stopping heating steam extraction on a low pressure cylinder gap bridge pipe; calculating the main steam flow, the cold reheat steam flow and the reheat steam flow; calculating the low pressure cylinder efficiency of a unit; taking a high back pressure heat supply unit as a pure condensing unit running under a high back pressure working condition, calculating the heat loss efficiency of the unit, and performing the second-type correction on the heat rate; and comparing the heat rate and the lower pressure cylinder efficiency with design values of a manufacture plant, and evaluating the lower pressure cylinder improvement technology and the unit improvement effect. The method disclosed by the invention is easy and feasible.