A child safety seat includes a seat portion, a backrest assembly connected with the seat portion and including a first and a second portion slidably connected with each other and a third portion slidably connected with the second portion, and a backrest adjusting system carried with the backrest assembly and configured to provide a first stage of height adjustment where the first and second portions are locked with each other and the third portion is movable relative to the first and second portions for adjustment, and a second stage of height adjustment where the second portion is locked with the third portion and unlocked from the first portion so that the second portion and the third portion are movable in unison relative to the first portion for adjustment.