The invention discloses a method of calculating critical dissolved boron content of a primary circuit coolant in a nuclear reactor. The method includes the steps: comparing the critical state at the last time of actuation of the reactor with the estimated critical state at this time of actuation of the reactor to calculate the change amount of the reactivity, wherein the corresponding variable ofthe critical state at the last time of actuation of the reactor includes the dissolved boron content of the first primary circuit coolant, and the corresponding variable of the estimated critical state at this time of actuation of the reactor includes the dissolved boron content of the second primary circuit coolant; according to the critical state at the last time of actuation of the reactor andthe estimated critical state at this time of actuation of the reactor, calculating the reactivity of the dissolved boron content of the second primary circuit coolant through a column reactivity balance formula; and calculating the value of the dissolved boron content of the second primary circuit coolant at this time of actuation of the reactor by multiplying the reactivity of the dissolved boroncontent of the second primary circuit coolant by the corresponding boron differential value. The method of calculating critical dissolved boron content of a primary circuit coolant in a nuclear reactor can avoid the theoretical data error caused by the reactivity, and is more precise in the calculation.