LoRa technology is a lowpower LAN wireless standard, which is increasingly used in the agricultural internet of things due to its advantages of low power consumption and long communication distance. LoRa nodes can not only be equipped with sensors for environmental monitoring but also figure out received signal strength indication (RSSI) of each node by checking the quality of received data packets. RSSI value has a strong relationship with signal transmission intensity, communication distance and communication media. A farmland internet of things system was designed and developed based on LoRa technology, and the relationship between LoRa communication quality, transmission power, communication distance and sugarcane plant population density was preliminarily explored, and the feasibility of using RSSI value to predict sugarcane population density was evaluated. In the communication experiment, a total of 3000 RSSI data were collected at 20 different distances in the open field and two sugarcane fields. The results showed that under the transmission power of 15dB, the communication quality on sunny days was more stable than that of 10dB, 20dB, rain, dew and other humidity, and the sugarcane density obviously affected the communication quality. The main factors affecting the communication quality in sugarcane field were analyzed based on these data, and the model of LoRa signal strength and communication distance was preliminarily established, the tests showed that its R2 reached 0.9291. The results showed that the system designed can provide important guidance for sugarcane growth monitoring, management and yield prediction.