Reducing the tritium inventory is one of the most important objectives in starting up a fusion reactor. To achieve this, the sponge mechanism, also known as deuterium presaturation, can be used to treat the surface of the tritium-contacting components prior to installation in the tritium recycling system. This treatment alleviates tritium from embedding in the material surface and reduces tritium retention in the reactor. Using a simplified physical model, the effect of the sponge mechanism on the minimum startup tritium inventory was simulated. Our results showed that the sponge effect can reduce the minimum startup tritium inventory by about 22%, shorten the tritium well time effectively, and extend the operational time determined by the tritium supply. Investigating the sponge mechanism is of great significance for designing a tritium recycling system.