The invention discloses a method for determining heat loads of a condenser of a condensing steam turbine. The heat loads of the condenser of the condensing steam turbine can be conveniently determined. Large steam turbine generator unit cold-side optimization work is carried out, and monitoring of the change of the heat loads of the condenser is one of important means. The method is characterized in that the thermal performance of a steam turbine generator unit is utilized, a whole steam turbine generator unit thermal system serves as an object, and according to an energy conservation principle, the heat loads of the condenser are directly acquired. A conventional method for calculating the heat loads of the condenser regards the condenser as the object, and compared with the conventional method, errors caused by calculation or measurement are avoided. A relation curve of the heat loss rate and the loads of the condensing steam turbine is adopted, operating parameters (modes) affecting the heat loss rate of the steam turbine are reversely corrected through a diagram or a formula, the operating heat loss rate of the steam turbine is acquired, and then the corresponding condenser heat loads are acquired.