The invention relates to the field of steam turbines, in particular to a calculating method for low pressure cylinder efficiency of a condensing steam turbine. The calculating method comprises the following steps that thermodynamic system parameters are measured; the enthalpy value of a steam water flow is obtained; the feedwater flow is obtained; corrected various shaft gland steam leakage is calculated; the low pressure cylinder ideal enthalpy drop and the steam turbine ideal enthalpy drop are obtained; the regenerative steam extraction capacity of a high pressure heater and a deaerator is calculated, the condensate flow of an inlet of the deaerator is calculated, and the regenerative steam extraction capacity of a low pressure heater is calculated; the work applying losses of the various shaft gland steam leakage and the regenerative steam extraction capacity are determined; the high pressure cylinder efficiency and the medium pressure cylinder efficiency are obtained; the actual internal power of the steam turbine is calculated, the ideal internal power of the steam turbine is obtained, and the relative internal efficiency of the steam turbine is calculated; and the low pressure cylinder efficiency is obtained. Obtaining of the low pressure cylinder efficiency is converted into obtaining of the ideal enthalpy drop and the relative internal efficiency of the steam turbine, measure points needed for testing are reduced, the calculating process is simple, the precision is high, and the engineering demands can be completely met.