Plant light response curves reflect dynamic charateristics of photosynthesis with the photosynthetic active radiation (PAR). The data collection for the curves is time-consuming and the photosynthesis is sensitively affected by surrounding environmental factors, so it is vital to establish suitable time interval for data determination in one day. Muskmelon is top ten most popular fruit in the world. However, researches on dynamic characteristics of photosynthesis in muskmelon seedling leaves were rarely reported. In order to probe into the relationships between characteristics of muskmelon photosynthesis in light response and environmental factors, suitable time intervals for measuring light response curves were optimized with potted muskmelon seedlings as test material, and the light response curves and several parameters of photosynthesis for different position leaves and environmental factors were measured by LI-6400 portable photosynthesis system from 18 October to 23 October, 2009. Each leaf position was measured at 3 intervals of time:10:00-12:00, 12:00-15:00 and 15:00-17:00 in one day. All of light response curves were fitted with the modified rectangular hyperbola model, which was widely applied and could fit light response curves that photosynthesis rate decreased obviously under intensive irradiation. Maximum net photosynthetic rate (P_max) was selected as an indicator of photosynthetic capacity. Direct and indirect effects of environmental factors on P_max were calculated through the path analysis. The results show that when photosynthetic active radiation in environment increased, leaf temperature (Tl) rose, and air relative humidity (RH) decreased; photosynthetic active radiation in environment decreased, Tl decreased, and RH increased. Light response curves between 10:00-12:00 and light response curves of leaf 1-4 between 12:00-15:00 were characterized by hyperbolic curves and the photosynthetic capability approached saturation under intensive irradiation, while photosynthesis rate of leaf 5 between 12:00-15:00 and photosynthesis rate between 15:00-17:00 decreased obviously under intensive irradiation. P_max and light saturation point (LSP) were the highest between 10:00-12:00 and declined between 12:00-17:00 during one day. Light compensation point (LCP) and dark respiration (R_d) were higher between 12:00-15:00 than that under the other two time intervals. Quantum efficiency at light compensation point (φ_c), stomatal conductance (G_s), and intercellular CO₂ concentration (C_i) all generally displayed a declining trend, however, stomatal limitation value (L_s) increased from 10:00 to 17:00. Stomatal limitation was the key factor for different time intervals influencing photosynthesis. Light responsive parameters of leaf 4-5 were positive at the same time interval between 10:00-15:00, the P_max of leaf 3 was the highest, leaf 5 was the second between 15:00-17:00. G_s and C_i of leaf 5 were lower than that of leaf 1, while L_s of leaf 5 was higher than that of leaf 1 between 10:00-17:00. Non-stomatal limitation was the major factor of influencing photosynthesis for different position leaves. RH was the primary determining factor that influenced P_max; measuring time (Time), vapor pressure deficit at the leaf surface (Vpdl) and Tl were main limiting factors, so it was helpful to improve P_max by increasing RH, limiting Time, Vpdl, and Tl. The suitable time period for measuring light response curves was between 10:00-12:00.