A child safety seat includes a backrest assembly including a first, a second, and a third portion slidably connected with one another, 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 so that the second and third portions are movable in unison relative to the first portion for adjustment. The third portion carries an adjustment latch that engages with the second portion for locking the third portion in position in the first stage of height adjustment and engages with the first portion for locking the third portion in position in the second stage of height adjustment.