Abstract: A child safety seat includes a base, a seat body and an engaging mechanism. The seat body has a rib structure. The rib structure is movably disposed in a guide-slot structure of the base so that the seat body is capable of sliding relative to the base. The engaging mechanism includes a first engaging part and a driving device. The first engaging part is movably disposed on the seat body for engaging with a tooth-shaped structure of the base. The driving device is movably connected to the first engaging part and is movably installed on the seat body so that the driving device is capable of moving between a releasing position and a locking position relative to the seat body. Thus, when the driving device moves relative to the seat body, the driving device can drive the first engaging part to alternatively engage with or separate from the tooth-shaped structure.