Abstract: The method to construct the dynamic differential equations of radionuclide concentration in primary loop was introduced in the paper. The data detected from coolant in primary loop were assigned to the equations’ initial value and the parameters of the equation could be adjusted by actual operation to reflect the real change of the concentration. The equations whose parameters were based on the actual operation situation from typical PWR were proved applicable compared with the results calculated by the current existing algorithm Profip5. Through analysis of the concentration of radionuclides calculated by the equations, the equilibrium value and the trend of concentration in primary loop and auxiliary systems were shown. The regular pattern of change for radionuclide concentration in primary loop under the cold down condition was also revealed in this paper. The results show that the calculated values of dynamic differential equations agree well with the results from Profip5, and the purification rate of radionuclides in primary loop by the chemical and volume control system conforms to the national standards. That means the equations can be used to calculate the concentration of radionuclides in the coolant. When detecting the fuel element failure, calculated concentrations have important significance to ascertain the time when failure happens and deduct the maximal value of actual concentration, the burnup area and size of the break.