Abstract: In recent years, the research on the automatic performance testing technology for drilling fluids has achieved significant progress both domestically and internationally. However, automatic drilling fluid testing instruments face challenges, such as high costs, complicated structures, and restricted working environments. Given these challenges, this study developed an automatic online testing instrument for the funnel viscosity and density of drilling fluids. The testing instrument consists of mud pumping, testing, cleaning, and control modules, which are built using a programmable logic controller (PLC), an Internet of things (IoT) module, a pressure sensor, an ultrasonic sensor, a peristaltic mud pump, an electric ball valve, and a diaphragm pump. It can fulfill functions including the automatic funnel viscosity and density testing of drilling fluids, and the data viewing and operation at the mobile terminal. This study enhanced the accuracy and stability of the testing instrument through sensor data fusion and filtering algorithms. The testing instrument was employed to test the performance of various drilling fluid systems, yielding funnel viscosity and density errors controlled at ±1 s and ±0.01 g/cm³, respectively. Furthermore, it allowed for continuous automatic performance testing for drilling fluids. The laboratory and field testing results suggest that the testing instrument boasts convenient operations, accurate testing results, high repeatability, minimal manual measurement errors, and labor saving. Therefore, it meets the demand for performance testing of drilling fluids in practical drilling construction, demonstrating high universality and great potential for application.