A seat fastening device is adapted to a child safety seat. The child safety seat includes a base and a seat body pivoted to the base via a pivotal shaft, so that the seat body can switch between a forward-facing orientation and a rearward-facing orientation by rotating around the pivotal shaft. The seat fastening device includes a locking component, a locked component assembled with one of the seat body and the base, and a tightening and loosening mechanism assembled with the other of the seat body and the base and connected to the locking component. The tightening and loosening mechanism is configured to loosen or tighten the locking component as the seat body is rotated to the forward-facing orientation or the rearward-facing orientation, so as to lock the locking component with the locked component, which achieves firm connection of the base and the seat body and convenient and rapid operation.