To explore effective measures for improving the cold tolerance of cotton seedlings, cotton cultivar ‘Xinluzao 61’ was used as the experimental material. We investigated the effects of foliar application of ascorbic acid(AsA) at four concentrations (0, 50, 100, and 200 mg/L) on major growth and developmental indices (plant height, leaf area, leaf relative water content, and dry matter content), photosynthetic parameters (Pn, Gs, Tr), and chlorophyll fluorescence parameters (PIABS, Fm/F0, Fv/Fm, ABS/RC) in cotton seedlings under 10 °C low-temperature stress. The results showed that, at different stress durations (0, 1, 3, 5, and 7 d), the growth and developmental indices as well as photosynthetic and fluorescence parameters of cotton seedlings treated with AsA were superior to those of the control (without AsA application), and 50 mg/L AsA exhibited the optimal effect. Compared with the indicators on day 0, cotton seedlings treated with 50 mg/L AsA for 7 days under low-temperature stress showed increases of 14.59%, 23.24%, and 17.18% in plant height, leaf area, and dry matter content, respectively. The main photosynthetic parameters(Pn, Tr) and Fv/Fm remained at relatively high levels during the first 3 days of low-temperature stress. In conclusion, exogenous application of an appropriate concentration of AsA can maintain high photosynthetic capacity of cotton seedlings, promote their growth and development under low-temperature stress, and thereby enhance the cold tolerance of cotton seedlings.