The invention discloses a control agent for gummy stem blight of watermelon at a growing period. The control agent comprises the following raw materials in parts by weight: 7-10 parts of common carpesium fruit, 11-14 parts of betel nut, 5-8 parts of coptis, 1.5-3 parts of cuttlebone, 1-2 parts of cattail pollen, 8-10 parts of ricepaperplant pith medulla tetrapanacis, 0.5-0.8 parts of cinnabar, 0.6-1 part of calamine, 11-13 parts of Red Psychotria Leaf, and 2-2.4 parts of hydrargyri oxydum rubrum. Through multi-year test, the control agent can be used for seed soaking, planting and root dipping, sprinkling and irrigation water usage, the methods can be diversified, the control agent has the advantages of safety and no side effect, is extracted by pure Chinese herbs, and has no harm and no residue for watermelon and farmland, through scientific compatibility, the control agent has special efficacy on ascomycotina and oomycetes, and can effectively inhibit the disease propagation at a diseases period, disease can be controlled by continuous application of the control agent for 3-5 times, and the control agent has alleviation effect for controlling various fungi diseases.