The invention belongs to the technical field of condenser monitoring, and particularly discloses a condenser dirt degree online monitoring method. The method includes the steps that a steam turbine off-design performance model is built, condenser steam parameters are obtained according to the steam turbine off-design performance model, and the overall heat exchange thermal resistance of a condenser is obtained through calculation according to the steam parameters of the condenser; the cooling pipe water side convection heat transfer thermal resistance of the condenser, the heat conduction thermal resistance of the wall of a cooling pipe and the steam side condensation heat transfer coefficient of the condenser are calculated, the steam side condensation heat transfer coefficient is modified, and the steam side condensation heat transfer thermal resistance is obtained; the dirt thermal resistance of the condenser is solved according to the overall heat transfer thermal resistance of thecondenser, the cooling pipe water side convection heat transfer thermal resistance, the heat conduction thermal resistance of the wall of the cooling pipe and the steam side condensation heat transfer thermal resistance, the dirt thermal resistance represents the water side cleanliness of the condenser, and then dirt degree online monitoring of the condenser is achieved. By means of the method, dirt degree real-time online monitoring of the condenser can be achieved, and the method has the advantages of being stable, accurate and tolerant to faults.