Abstract: The mechanism of tritium production and reduction in primary coolant of pressurized water reactor (PWR) was studied. The tritium calculation models were established based on tritium metabolism. The contribution of various ways to generate tritium was calculated and the influence of ⁷Li purity on tritium production was analyzed. The results show that the optimization models consider in detail the change of nuclides concentration from which tritium nuclides are produced with time, and the tritium production is 52.08 TBq/a. The tritium nuclides in PWR primary coolant are mainly from the activation reaction of soluble boron by absorbing neutron and the ternary fission of uranium, which contribute more than 90%. When the purity of ⁷Li is 99.9%, the tritium produced from lithium accounts for 7.45%. Other ways produce a small amount of tritium which can be ignored.The contribution of lithium decreases linearly when the purity of ⁷Li increases. The research results could provide a reference for the calculation of tritium source term of PWR.