Abstract: The interior space of a typical private vehicle is limited and somewhat sealed, which can lead to a rapid increase in temperature, especially during hot weather. This condition poses a risk of heatstroke and potentially life-threatening situations if individuals are stranded inside the vehicle. In this paper, an in-vehicle temperature monitoring and remote positioning warning device is designed. The device performs in-vehicle temperature detection based on detecting the presence of a person stranded in the vehicle. When the temperature exceeds a predefined threshold, a remote warning is triggered, and the vehicle is quickly located to facilitate prompt retrieval by the owner. The design process begins with the structural modeling of the device using Unigraphics NX software, enabling integration and loading of sensors. The device is designed to be detachable. Subsequently, the hardware circuit is designed using Altium designer, and the programming is implemented using Keil to enable the detection of occupants and temperature inside the vehicle. Furthermore, a GPS positioning module is utilized for precise vehicle localization, and remote communication is established through a 4G module, the Ali Cloud platform, and a mobile app to enable data transmission and warning notifications. The device adopts an external design and connects to the vehicle via USB, without intruding into the central control system, thus not affecting the vehicle’s original circuits or inherent safety.