A backrest translation mechanism includes a holder mounted on a sofa seat, a swing locating member, a backrest mount, a slider, and a positioning pin. The swing locating member has a first end pivoted on the holder, several locking slots with downward openings arranged at regular intervals along its length direction. The locking slots are adapted for engaging with and unidirectionally locking up the positioning pin, one side of each locking slots is provided with an inclined surface inclined towards an opposite direction of the first end. The slider is slidably set on the swing locating member and has a retaining slot and a limiting slot. The positioning pin is accepted by the retaining slot and the locking slot jointly, and the positioning pin is restricted to be out of the locking slot by the limiting slot. The mechanism has multiple using positions for adjusting the translation of the backrest.