A three-dimensional (3-D) printer and a method for adjusting a working coordinate of a platform thereof are provided. The 3-D printer includes a platform, a printing head and a control unit. The platform includes a carrying surface and adjustment points located on the carrying surface. The printing head disposed above the platform for moving along a datum plane and a normal direction of the datum plane. The control unit controls the printing head to move from the datum plane toward the platform to contact each of the adjustment points for obtaining a coordinate offset of the carrying surface corresponding to the datum plane, and adjusts a model coordinate of a digital 3-D model information according to the coordinate offset. The control unit moves the printing head according to the adjusted model coordinate to print a 3-D object related to the digital 3-D model information on the carrying surface.