Abstract: Damage equivalent is an important index to measure the severity of reactor fuel element damage. However, the damage equivalent is not operable in the decision-making application because it cannot be directly measured. It is necessary to establish the monitoring index corresponding to the damage equivalent. Based on practical experience, the typical nuclides that can be used for damage diagnosis are analyzed and determined, and the transfer relationship between the activity concentration of fission product nuclides in the reactor primary circuit coolant and the damage equivalent of fuel elements is established; the experimental method of sampling and analyzing the primary circuit coolant is given, and the issues that should be paid attention to in the experimental process are pointed out; a method for diagnosing the damage equivalent by monitoring the nuclide activity concentration of typical fission products in the primary circuit coolant is established, and the main factors affecting the uncertainty in the diagnosis are analyzed. This study provides a technical method for diagnosis of reactor fuel element damage equivalent.