Magnetic fields are generally sensed by a device that makes use of the Hall effect. Hall-effect sensors are widely used for proximity switching, positioning, speed detecting for the purpose of control and condition monitoring. Currently, the Hall sensor products are mainly based on the narrow-bandgap Si or GaAs semiconductor, and they are suitable for room temperature or low temperature environment, while the novel wide-bandgap GaN-based Hall sensors are more suitable for the application in various high-temperature environments. However, the spatial structure of the GaN-based sensor is mainly horizontal and hence it is only able to detect the magnetic field perpendicular to it. To detect the parallel field on the sensor surface, the vertical structure device is required despite encountering many difficulties in technology, for example reducing the vertical electric field in the two-dimensional electron gas (2-DEG) channel. The vertical Hall sensor has not been reported so far, so it is technically impossible to realize three-dimensional magnetic field detection on single chip. To address the mentioned issues, in this paper we propose a design of the vertical Hall sensor based on the wide-bandgap AlGaN/GaN heterojunction material, which adopts a shallow etching of 2-DEG channel barrier to form a locally trenched structure. The material parameters and physical models of the proposed device are first calibrated against real device test data, and then the key structural parameters such as device electrode spacing ratio, mesa width and sensing electrode length are optimized by using technology computer aided design, and the device characteristics are analyzed. Finally, the simulation results confirm that the proposed Hall sensor has a higher sensitivity of magnetic field detection and lower temperature drift coefficient ( $\sim $ 600 ppm/K), and the device can work stably in a high-temperature (greater than 500 K) environment. Therefore, the vertical and horizontal devices can be fabricated simultaneously on the same wafer in the future, thus achieving a three-dimensional magnetic field detection in various high-temperature environments.