Nanomechanical resonant sensors based on two-dimensional materials have great potential in sensing with the advantages of small size, high frequency, and high-quality factor. Among them, black phosphorus provides a unique opportunity for in-plane vector sensing due to its corrugated planar structure and anisotropic mechanical characteristics. In this paper, a new type of a nano-opto-electromechanical magnetic vector sensor based on optically excited optical readout is designed using black phosphorus. Using the finite element analysis method, the influences of the number of layers and the length-width ratio of black phosphorus on the vector property are studied, and the system structure is optimized with the size of 0.6 μm×0.134 μm×0.5 nm. The anisotropy of the response to the magnetic field in the directions of high Young’s modulus and low Young’s modulus is explored, respectively. By using the spatial distribution of resonant modes, the influences of the angle and length-width ratio on the sensitivity and vector property of the device are explained. It can be found that in the Armchair direction, when the magnetic field rotates 90°, the sensitivity changes from -4.048 MHz/mT to 5.796 MHz/mT. Compared with Lorentz force-based micro-electromechanical sensors, the resonant frequency of the designed sensor can be improved by 6 orders of magnitude, and its size can be reduced by 6 orders of magnitude. This design provides a new method for the preparation of a nano-opto-electromechanical vector sensor.