In previous studies on recording the low-frequency holographic grating via the Mach-Zehnder interferometer, a detailed analysis of the width and contrast of the interference fringes has revealed their rules of variation. This paper focuses on the fringe orientations for recording by the Mach-Zehnder experiment. It is confirmed experimentally and theoretically that the direction slope of the interference fringe orientations k1 is dependent on that of the connection line of the point light source k2 of two interference light, with a relation of k1×k2=-1. This result provides a theoretical and experimental basis for recording the different fringe-orientation low-frequency holographic grating and their applications in precision measurements.