A watermelon maturity monitoring device is mainly composed of a detector (1), an elastic sling (2), a net bag (3) and a support (4). The watermelon maturity monitoring device is characterized in that the detector (1) is installed on the elastic sling (2) and stretches and contracts along with the elastic sling (2), when matured, a watermelon falls due to weight, and the elastic sling (2) rebounds and triggers the detector (1) to act to remind people of watermelon maturity; one end of the elastic sling (2) is connected with the net bag (3) containing the watermelon, and the other end of the elastic sling (2) is connected with the support (4). The mode of monitoring watermelon maturity through weight change is lossless watermelon monitoring, and meanwhile watermelon maturity is monitored in time in the watermelon growth process and is judged accurately in time. The watermelon maturity monitoring device is first application of the weight method monitoring technology in the watermelon growing process and has the advantage of creativity.