A three dimensional printer includes a spray head, a transmission mechanism, a construction platform, an adjustment device of the construction platform, a plurality of oil cylinders, an oil tank and a sensor. The spray head is disposed opposite to the construction platform, and the transmission mechanism is connected to the spray head for driving the spray head to move at a horizontal displacement plane. The oil cylinders are disposed at the construction platform for adjusting the construction platform to be parallel to the horizontal displacement plane. Hydraulic oil is stored in the oil tank, and each oil cylinder is connected with the oil tank through an oil pipe. The sensor is disposed at the spray head for sensing a plurality of vertical distances between the spray head and the oil cylinders. Therefore, the adjustment process and mechanism are simplified, and the advantages of enhancing the precision of the adjustment are achieved.