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Sap flow is an important physiological index of plants. So it is necessary to conduct long-term, accurate and online monitoring of sap flow in the study of water consumption law of transpiration and related applications. In view of the high price of foreign commercial sap flow meter, the modular sap flow meter has many problems in terms of volume, performance, sap power supply and communication, and a portable plant sap flow monitor based on the principle of thermal dissipation was developed by using the integrated design concept. The embedded microprocessor STM32 was taken as the main control module, the AD620 and OP07 composed of low noise precision amplification circuit was taken as the core, and the probe constant current heating, SD card storage and GPRS communication, solar power supply circuit and power monitoring alarm were designed. The maximum measured voltage of the day was obtained by iterative algorithm, and the plant sap flow density was calculated immediately. On-site test and comparative test showed that the instrument had displaying, storing, reading, transmitting and alarming functions normally, the measured sap flow density data was accurate, the performance was stable, and the results obtained by the comparison instrument Delta-T data collector had good consistency. In the dualchannel monitoring, the working time was about 7 days in continuous rainy weather, and it can work for long periods of time under normal weather conditions. The sap flow meter had the advantages of cost-effective, easy to use, easy channel expansion, and can replace imported products. It had good application prospects in the field of plant water transmission and utilization research, the monitoring of the impact of forest ecosystems on environmental changes, and the water management of urban green space.