A child safety seat includes a seat body, a backrest, a first side wing, and an adjusting mechanism. The backrest is connected to the seat body. A hole is formed on the backrest and a first slot is formed on a first side of the backrest. A first side wing is movably disposed through the first slot. The adjusting mechanism is movably disposed in the hole of the backrest and is movably connected to the first side wing. The adjusting mechanism has an actuating part. The actuating part is operable for driving the first side wing to move relative to the first slot telescopically.