This study measured physiological factors of Ulmus pumila, namely the net photosynthetic rate (P_n), upper epidermis stomatal conductance (G_s-u), and lower epidermis stomatal conductance (G_s-l), in Otinday Sandland (42°23'N, 115°37'E) of Inner Mongolia using Li-6400 portable photosynthesis system. Ecological factors, namely the atmospheric temperature (T_a), photosynthetically active radiation (PAR), concentration of carbon dioxide in the atmosphere (C_a), and relative humidity of the air (RH) were also examined. Based on the measured data, the relationship between P_n diurnal variation in U. pumila at various growth periods and ecological factors and physiological factors was assessed using correlation analysis, path analysis, decision coefficient analysis, and grey correlation analysis. This knowledge is essential to reveal the physiological characteristics and formulate generalizations regarding the cultivation of U. pumila in Otinday Sandland. Regardless of growth period, P_n exhibited bimodal diurnal variation with varying degrees of photosynthesis "midnaps." P_n was highest in August (8.18 μmol CO₂ m^-2 s^-1), lowest in September (8.18 μmol CO₂ m^-2 s^-1 versus 6.52 μmol CO₂ m^-2 s^-1), and intermediate in July (7.92 μmol CO₂ m^-2 s^-1). In the three months from July to September, P_n was found to have significant positive correlations with PAR, which had the greatest direct influence and thus the largest decision coefficient, and G_s-u, which had the greatest indirect influence but a smaller decision coefficient than PAR. The other factors exhibited varying degrees of influence on P_n. The relative influence of ecological factors and physiological factors on P_n depended on the growth period. T_a and RH were the main controlling factors in July and August, respectively, and all factors had positive contributions. Although P_n was mainly influenced by PAR and G_s-u, regardless of growth period, a simple linear functional correlation was not found owing to interactive effects.