A perfect zero reference mark is essential for an incremental measurement system to achieve absolute coordinates. An optical zero reference mark based on a phase jump is presented. The zero reference position signal is achieved using two same-frequency focused beams to cross an edge on a simple scale. The phase difference between these two beams is prefixed at 180 deg. The use of a heterodyne interference technique enables the phase jump to be detected. The zero signal is a phase jump of 180 deg, and the theoretical slope of the phase jump is infinite. Because the phase differences of the signal on two sides of the phase jump are 180 deg, the direction can be also obtained. The zero signal is high in resolution and has excellent disturbance resistance. At the same time, the zero reference mark can be naturally integrated with a commercially available positioning system. Both the theoretical design and experimental verification are conducted and the results are presented in detail.