1. A method for tracking positions of human extremities, comprising: retrieving a left image of a first extremity portion using a first picturing device; obtaining an outline candidate position of the first extremity portion according to feature information of the left image; retrieving a right image of the first extremity portion using a second picturing device; obtaining a depth candidate position of the first extremity portion according to depth information of the right image; calculating geometry relations between the outline candidate position and the depth candidate position and a second extremity position of a second extremity portion; and determining whether a current extremity position of the first extremity portion is required to be updated.