As novel sensors, rotational symmetric triangulation sensors (RSTSs) have broad application potential because of their robustness and accuracy. However, existing RSTSs have the disadvantages of complicated structure and high cost. The current paper presents a novel RSTS based on an object space mirror. The proposed sensor uses a cone mirror and a commercial camera, which significantly simplifies the structure and reduces its cost. Mathematical models are discussed to reveal the measurement range of the sensor and the relation between the displacement of the spot and the radius of the ring in the image. An improved image processing method is also introduced for a more efficient processing. Finally, two experimental systems with mirror angles at π/6 and π/4 are set up. Experimental results showed that the proposed sensor can achieve a comparable accuracy of 1.6 μm compared with that of existing methods, as well as a higher efficiency of image processing, while remaining at a low cost.