In order to research the tritium aging effect in hydrogen deuterium exchange at a constant flow rate between gaseous hydrogen and palladium hydride powder, the reasons that affect exchange performance were analyzed by speed theory of chromatography separation, and the breakthrough curves of the exchange reaction were simulated by plate theory. And, hydrogen deuterium exchange experiments over 0 year and 1.6 years aged palladium were compared. The theoretical simulation results indicate that in aged palladium, the plate number of exchange reaction is decreased, the plate height is increased, the equilibration time of exchange reaction is prolonged, the breakthrough curves become smooth, and the exchange performance declines. The plate height increasing is due to decay product 3He, which accumulate and cumber hydrogen isotope diffusion in aged palladium; further more, because of the existence of 3He, the hydrogen isotope separation factor of aged palladium changes. The comparison results indicate that the hydrogen deuterium exchange breakthrough curve of 1.6 years aged palladium go smooth, and the hydrogen deuterium exchange performance declines at a constant hydrogen flow rate. The theoretical simulation results accord with the experiment results.