A set of crop root growth monitoring minirhizotron device was designed and developed to achieve real-time monitoring and image collection of crop roots at different depths. The collected images can be displayed and stored in real-time through upper computer software. The device consisted of a monitoring tube and a control box. The monitoring tube used acrylic material to make a transparent shell, and a sliding track was set up inside the tube through screws and guide rods. The motion control of the camera on the track was achieved by using a stepper motor. The control box was based on the STM32 microcontroller as the core control board, and corresponding peripheral modules were selected according to actual needs. The tomato root system was used as the research object for 98d of image collection, and the root image was analyzed by using the software RhizoVision Explore. The experimental results showed that the overall growth rate of tomato roots was faster in the first 70d, and gradually stabilized in the last 28d, and distributed densely at depths of 6~10cm. The root length density reached its maximum value of 1.22cm/cm3 in the 91st day at a depth of 10cm. The analysis results were consistent with the growth pattern of tomato roots, indicating that this root growth monitoring minirhizotron device can complete long-term online monitoring of crop roots without affecting the continuous growth of roots, meeting the requirements of crop root monitoring.