Based on tritium production mechanism and the tritium calculation model in China General Nuclear Power Corporation (CGN), the historic tritium discharges in lots of nuclear power plants around the world have been comprehensively gathered and analyzed. It is recognized that the activation of boric acid in the primary loops and activation of Be in secondary neutron source (40% of annual tritium discharge in CGN’s operating pressurized water reactor units) are the predominant origins, and the tritium release from intact fuel rods with zirconium alloy can be neglected. It is found that use of double encapsulated secondary neutron source or cancellation of secondary neutron source is the only way to reduce significantly the tritium discharges in the pressurized water reactors.