An automatic replenishing mechanism for a pet drinking fountain is a container having a water injection element, a float, a linkage set, and a sealing plug. The water injection element, connected to the container, has a water inlet and a base. The float, connected by a link bar to the base, is located underneath the water inlet and limited by a guiding portion to float longitudinally. The sealing plug is mounted on the link bar near the water inlet. When water level rises to elevate the float, the link bar rotates to push the sealing plug. When the float floats to a predetermined level, the sealing plug blocks the water inlet to stop water supply. When a pet drinks water and the water level drops, the float sinks from the predetermined level. The link bar rotates to retreat the sealing plug from the water inlet, thereby automatically replenishing water.