A portable electronic device for measuring a distance D1 between two points E1, E2, includes a visible and rotatable light emitter, an angle computing unit, a distance computing unit and an output unit. The light emitter generates a rotatable light beam capable of rotating from pointing in an initial direction to project to the point E1. The initial direction is substantially perpendicular to a line defined by the points E1, E2. A distance H1 between the light emitter and the line, and a distance S1 from the point E2 to the point of intersection of the line defined by the first and second points E1, E2 with the initial direction, are known. The angle computing unit computes an angle defined by the visible light beam projecting to the point E1 and the initial direction according to the rotatable light beam. The distance computing unit computes the distance D1 based on the angle, the distance H1 and the distance S1.