Objective: In order to meet the demand of low water-content fruit-vegetable powder online detection, a new detection system was established. Methods: Based on the microwave transmission principle, an online moisture detection system of fruit-vegetable powder was designed. The system was divided into a feed module, a detection module, a discharge module and control display module, and the corresponding hardware and software design. Carrot powders were used as the test material, and a third-party testing institution compared the two methods, including water-content microwave transmission and national standard testing, to evaluate the test performance of the device. Results: The moisture content of each sample was determined to be about 7.33 s by the new device, with an accuracy of 98.2% and repeatability of 0.15%. Conclusion: The system-based device can realize online and continuous detection of fruit-vegetable powder with low water content.