Abstract: In view of the common flue-cured tobacco rotation system in Southern China, the decomposition characteristics of three crop straws (tobacco, rape and rice) and the effects of the straw decomposed liquids on Phytophthora nicotianae were studied in order to provide theoretical basis for comprehensive utilization of tobacco field straws. Simulation experiment was adopted by using nylon net bags to study the decomposition rates and nutrient release patterns of three crop straws under two soil moisture conditions, and the effects of different concentrations of decomposed liquids on hypha growth and sporangium yield of Phytophthora nicotianae were analyzed by artificial preparation of three kinds of straw decomposed liquids. The results showed that tobacco straw and rape straw decomposed faster at the early stage, but the overall decomposition rate was shown as rice straw > rape straw > tobacco straw. The cumulative decomposition rate of straw in each treatment was above 55% in 180 days. The nutrient release rates of three crop straws were all shown as K > P > N, 60.65%-70.18% of nitrogen, 75.33%-84.81% of phosphorus and 93.96%-97.34% of potassium were released during decomposition. The decomposition rate and nutrient release rate of straw in dry-wet alternate treatment were higher than those in flooding treatment, but the differences were not significant. The results also showed that the three straw decomposed liquids had inhibition effects on hypha growth and sporangium yield of Phytophthora nicotianae, the inhibition rates were shown as rice straw > tobacco straw > rape straw. And the inhibition effect was enhanced with the increase of decomposed liquids concentration, with the median effective concentration (EC₅₀) being 0.82, 1.40 and 2.08 g/L, respectively. Therefore, rice straw was easy to decompose in tobacco rotation system, and the three straw decomposition liquids had allelopathic effect on Phytophthora nicotianae. The effects of different water conditions on straw decomposition and nutrient release efficiency need to be further studied.