1. A power management method for a handheld electronic device, comprising: using a G-sensor for detecting a normal vector of a plane which is a screen of the handheld electronic device comprising a verticle type or a glide open type handheld electronic device; determining whether the normal vector comprising an x, y, or z component is directed toward a downward direction by determining whether the z component of the normal vector is negative, and the x component and the y component of the normal vector fall within a specific range respectively; and the normal vector is determined to be directed toward the downward direction when the z component of the normal vector is negative and the x component and the y component of the normal vector fall within the specific range; and controlling the handheld electronic device to enter a power saving mode if the normal vector of a plane which is a screen of the handheld electronic device is directed toward the downward direction.