A 3D model construction device includes a camera and a wearable display coupled to the camera. The camera obtains multiple first frames, a second frame and depth information. The wearable display includes a display unit, a processing unit, a storage unit and a projection unit. The storage unit stores a first module and a second module. When the first module is performed by the processing unit, the processing unit calculates a first pose of the wearable display. When the second module is performed by the processing unit, the processing unit calculates a 3D model according to the first frames, the depth information, the first pose and calibration parameters, and updates the 3D model according to the second frame. The projection unit projects the 3D model and the second frame onto the display unit according to the first pose for being displayed with a real image on the display unit.