The invention discloses a method for estimating the exhaust enthalpy of a low pressure cylinder of a steam turbine in real time, which belongs to the field of steam turbine parameter measurement. The method comprises: firstly, defining the exhaust efficiency of the (i+1)th stage of a turboset; secondly, obtaining the exhaust enthalpy of the (i+1)th stage of the turboset accordingly; and thirdly, according to a collected heat balance diagram of the steam turbine, obtaining the design parameters of the steam turbine under various working conditions, fitting the relationship between inter-stage efficiency of steam turbine and pressure ratio and calculating the exhaust efficiency eta i, i+1 of the (i+1)th stage of the seam turbine under different conditions from the exhaust efficiency of the last stage upward stage by stage. The method overcomes the drawback that the calculation speed of a loop iteration algorithm is far insufficient for meeting the demands for on-line monitoring of the machine and estimates the exhaust enthalpy of the low pressure cylinder of the steam turbine in real time when the turboset works under various working conditions.