Abstract: To address the issues of power system operation failures in outdoor substation terminal boxes caused by condensation, moisture ingress, and abnormal door opening, traditional manual inspection methods suffer from drawbacks such as long cycles, low efficiency, and susceptibility to missed inspections. This paper develops an intelligent condition monitoring device for outdoor terminal boxes based on real-time monitoring and fusion judgment of multiple status parameters. The device collects internal temperature, humidity, and door status in real time using high-precision RTD temperature sensors, capacitive humidity sensors, and inductive proximity switches. An STM32 microcontroller serves as the core processing unit, transmitting data to the cloud platform via an NB-IoT wireless communication module. The backend calculates a condition assessment value using a parallel logic judgment algorithm and implements an audio-visual alarm through a mobile App. Field tests demonstrate that the device can accurately identify abnormal conditions in terminal boxes with an alarm response time of less than 3 seconds. It effectively enhances the timeliness and reliability of terminal box status perception, reduces operation and maintenance costs and safety risks in substations.