The early-maturing cotton cultivar Huamian 3097 was used to study the rational application dosage of nitrogen for the direct seeding cotton after harvesting rapeseed oil in the cotton area of the Yangtze River Basin based on the concept of “increasing yield without increasing input”. Field experiments were conducted in 2022 and 2023 to investigate the effects of six application dosages of nitrogen including N₁₈₀ （180 kg/hm²）， N₂₁₀ （210 kg/hm²）， N₂₄₀ （240 kg/hm²）， N₂₇₀ （270 kg/hm²）， N₃₀₀ （300 kg/hm²）， and N₃₃₀ （330 kg/hm²） on the growth and development， photosynthetic characteristics， and yield of cotton. The results showed that the yield of cotton under N₂₇₀ and N₃₀₀ was comparable， but higher than that under N₁₈₀ and N₃₃₀.The two-year average yield of cotton seed and the number of bolls per unit area under N₂₇₀ increased by 7.0% and 5.4%， 24.9% and 21.6% compared to that under N₁₈₀ and N₃₃₀， respectively. There was no difference in plant height， number of fruit branches， height of the first fruit branch， number of green leaves， and number of nodes between treatments. The light transmittance of cotton canopy decreased with the process of growth and the increase of application dosage of nitrogen. The light transmittance of cotton canopy within the canopy height of 30-50 cm and the cotton row of 20 cm under N₂₇₀ was 23.9% higher than that under N₃₃₀， 10.0% lower than that under N₁₈₀， and comparable to that under N₃₀₀.The net photosynthetic rate， transpiration rate， and stomatal conductance under N₂₇₀ （N₃₀₀） was higher than that under N₁₈₀ and N₃₃₀. The concentration of intercellular CO₂ under N₂₇₀ was lower than that under N₁₈₀ and N₃₃₀. It is indicated that applying 270 kg/hm² nitrogen fertilizer to direct-seeding cotton after harvesting rapeseed oil is conducive to the growth and development of cotton， can increase the number of bolls per unit area and improve light transmission of canopy to enhance photosynthetic performance， and thus can achieve a higher yield.