A door closer capable of adjusting its closing speed, including a gas adjusting apparatus arranged on the door frame, a driving apparatus arranged on the door, and a lever is provided. One end of the lever is movably connected to the gas adjusting apparatus, and the other end is connected to the driving apparatus. The gas adjusting apparatus includes a sliding rail arranged on the door frame, and a sliding member cooperated with the sliding rail; a hermetic chamber is formed by the sliding member and the sliding rail, an adjusting member, for adjusting the exhaust of the hermetic chamber, is configured on a side wall of the hermetic chamber. The lever is movably configured on the sliding member.