A support base for a child safety seat includes a shell body and a belt restraining frame. The shell body is adapted to support a child seat, and has an upper surface provided with a storage cavity sized to receive an attachment strap and two latch connectors. The belt restraining frame is pivotally connected with the shell body, and is rotatable relative to the shell body between a release position where the belt restraining frame rises away from the upper surface of the shell body so that the storage cavity is opened upwardly, and a clamping position where the belt restraining frame is adjacent to the upper surface for holding an anchoring belt that is routed across the storage cavity.