A child safety seat includes a seat portion, a backrest assembly connected with the seat portion, and a backrest adjusting system carried with the backrest assembly and including a lock. The backrest assembly including a first portion, a second portion slidably connected with the first portion, and a third portion slidably connected with the second portion. The lock includes a first latching part operable to lock and unlock the third portion with respect to the second portion, and a second latching part operable to lock and unlock the first portion with respect to the second portion, the second latching part being coupled to the first latching part so that the first and second latching parts are movable in unison relative to the backrest assembly.