A three-dimensional printing device for printing a second object on a first object, the three-dimensional printing device comprising: a body; a three-dimensional printing unit, movably disposed on the body; a storage unit, storing initial coordinates, wherein the initial coordinates correspond to an initial printing position of the three-dimensional printing unit; a carrier plate (150,350) located at a bottom of the body, and having a ocular mark, wherein the ocular mark indicates a corresponding position of the initial coordinates; a sensor unit; a control unit, electrically connected to the three-dimensional printing unit and the body, the storage unit and the sensor unit, wherein the body has a plurality of guide rods (112a,112d) set on the bottom, a plurality of brackets (114a, 114b) sleeving the guide rods (112a-112d), a connection rod connected between the brackets (114a, 114b), and a plurality of driving units (118a to 118e), the brackets (114a, 114b) are adapted to move along a first direction perpendicular to the carrier plate, the driving units (118c, 118d, 118e) are configured for driving the connection rod and the three-dimensional printing unit to move relative to the brackets (114a, 114b) along a second direction perpendicular to the first direction, wherein the sensor unit is disposed on the brackets (114a, 114b) and moves along with the brackets (114a, 114b) and the three-dimensional printing unit in the first direction, the sensor unit is controlled by the control unit to sense a height of the first object relative to the carrier plate, and the control unit drives the three-dimensional printing unit to print the second object on the first object according to the height and the initial coordinates. a body; a three-dimensional printing unit, movably disposed on the body; a storage unit, storing initial coordinates, wherein the initial coordinates correspond to an initial printing position of the three-dimensional printing unit; a carrier plate (150,350) located at a bottom of the body, and having a ocular mark, wherein the ocular mark indicates a corresponding position of the initial coordinates; a sensor unit; a control unit, electrically connected to the three-dimensional printing unit and the body, the storage unit and the sensor unit, wherein the body has a plurality of guide rods (112a,112d) set on the bottom, a plurality of brackets (114a, 114b) sleeving the guide rods (112a-112d), a connection rod connected between the brackets (114a, 114b), and a plurality of driving units (118a to 118e), the brackets (114a, 114b) are adapted to move along a first direction perpendicular to the carrier plate, the driving units (118c, 118d, 118e) are configured for driving the connection rod and the three-dimensional printing unit to move relative to the brackets (114a, 114b) along a second direction perpendicular to the first direction, wherein the sensor unit is disposed on the brackets (114a, 114b) and moves along with the brackets (114a, 114b) and the three-dimensional printing unit in the first direction, the sensor unit is controlled by the control unit to sense a height of the first object relative to the carrier plate, and the control unit drives the three-dimensional printing unit to print the second object on the first object according to the height and the initial coordinates.