The opening mechanism includes a fixing member, a movable member, a locking member, a first resilient member and a second resilient member. The fixing member is fixed to a first rail and has a guide portion, a stop and a room. The movable member is movably connected to the fixing member and has a first leg, a second leg and an engaging portion. The first leg slidably contacts the guide portion and the second leg extends into the room. The locking member is pivotably connected to a second rail and includes a locking portion. The first and second resilient members respectively provide a force to a third rail and the movable member. When the second rail is forcibly pulled out relative to the first rail, the movable member is moved and pivots at an angle so that the locking portion is disengaged from the engaging portion.