Golden Ulmus pumila is a natural golden leaf mutant of U.pumila.The genetic analysis revealed that the inheritance of yellow-leaf phenotype belongs to cell nucleus and showed character separation in the next generation.Used the yellow seedlings and green seedlings as materials,the photosynthetic characteristics and leaf colouration machinsm were studied form the growth rate,pigment contents of leaves,photosynthetic rate,chlorophyll fluorescence and the ultrastructure of chloroplasts of leaves.The results showed:(1)The yellow seedlings developed slowly,and the net photosynthetic rate(12.5 μmol·m^-2·s^-1)was significantly lower than that of green seedlings(17.5 μmol·m^-2·s^-1),but the transpiration rate,stomatal conductance and leaf temperature were significantly higher than those of green seedlings.(2)The types of pigments were almost same,but the contents of 4 pigments of yellow seedlings were lower than those of green seedlings all the time and relatively stable.In the growth period,all kinds of pigment contents changed slightly in different months and the carotenoid content was always lower than the chlorophyll content.(3)The reduction of electron transfer efficiency through PSⅡof yellow seedlings was lower than that of green ones,(4)The inner membrane system of the chloroplasts developed incompletely and the grana stacked together imperfectly,which led to the reduction of the chlorophyll－protein complexes.Therefore,development defect in the endomembrane system of the chloroplast and the failure stack of the grana lamellae of yellow seedling led to the photosynthetic system developed incompletely,all kinds of pigment contents decreased in abundance,and then the color of leaves showed yellow,the photosynthetic capacity decreased and the seedlings grew slowly.