In order to explore the correlation between the varietal differences of monosaccharide and Phytophora parasitica var.nicotiana，we used ‘NC102’，‘K326’，‘Yun87’ and ‘Honghuadajinyuan’ as materials to study the monosaccharide in the root and rhizosphere soil of different fluecured tobacco varieties and its allelopathy to P. parasitica var.nicotiana. The results showed that: （1） xylose, arabinos, glucose and galactose were detected in the root and rhizosphere soil of all the fluecured tobacco varieties, and among them, glucose content achieved the highest level. （2） At rosette stage, the glucose in the rhizosphere soil of highresistant variety ‘NC102’ and the mediumresistant variety ‘K326’ are: 2.255 μg·g_-1 and 4.744 μg·g_-1, respectively; the glucose in the rhizosphere soil of mediumsusceptible variety ‘Yun87’and highsusceptible variety ‘Honghuadajinyuan’ are: 7.100 μg·g_-1 and 11.068 μg·g_-1, respectively; at the rosette stage, the order of total content of monosaccharide in the rhizosphere soil of all fluecured tobacco varieties is: ‘NC102’（5.346 μg·g_-1）＜’K326’（7.844 μg·g_-1）＜‘Yun87’（10.551 μg·g_-1）＜ ‘Honghuadajinyuan’ （15.579 μg·g_-1), respectively. （3） There is a tendency that the total amount of glucose and monosaccharide is high in the rhizosphere soil of susceptible varieties and low in the rhizosphere soil of resistant varieties. The total amount of monosaccharide is high in the rhizosphere soil of susceptible varieties at the rosette stage, and low at other stages. （4） Each monosaccharide and saccharidemixture has promotion effect on the growth of P. parasitica var. nicotiana, and the saccharide with the concentration of 2.000 mg·mL_-1 and the saccharidemixture with the concentration of 4.000 mg·mL_-1 both have the strongest effects. The study found that susceptible varieties of fluecured tobacco secrete more monosaccharides into the rhizosphere through the roots during the early stages of growth and development, providing materials and energy for the growth and development of P. parasitica var. nicotianal, thus promote its growth and lead to its occurrence.