A field experiment was undertaken to measure diurnal variation in photosynthetic and light-response parameters of leaves of Phragmites australis growing in the Liaohe River Delta wetland, China. Four developmental stages (leaf-expansion, jointing, heading, and mature stages) and three (upper, middle, and lower layers) or five vertical layers (top, upper, middle, lower, and bottom layers) were delimited. Diurnal variations in net photosynthetic rate, stomatal conductance, and transpiration rate showed single-peak or double-peak curves that were lower in the morning and evening and higher at noon. The diurnal variation in intercellular CO2 concentration showed the opposite pattern, with higher values in the morning and evening and lower values at noon. Midday depression was observed under strong light, in the top, upper, and middle layers but not under weak light or in the lower and bottom layers. The net photosynthetic rate, stomatal conductance, and transpiration rate were higher in the upper layer, and gradually decreased in value and in diurnal variability below the middle layer with increasing proximity to the plant base. Leaves showed a strong photosynthetic ability at the leaf-expansion stage, with the maximum net photosynthetic rate in the middle layer and the minimum net photosynthetic rate in the lower layer. Photosynthetic ability increased at the jointing and heading stages. The maximum net photosynthetic rate was in the upper or middle layers and the minimum in the bottom layer. Photosynthetic ability was weak at the mature stage. The light compensation point in leaves was 27.1–38.2, 24.6–29.5, 27.7–72.6, and 15.4–31.0 μmol⋅m−2⋅s−1 at the leaf-expansion, jointing, heading, and mature stages, respectively. The developmental stages of P. australis leaves were ranked, from highest ability to use weak light to lowest, as follows: mature stage > jointing stage > leaf-expansion stage > heading stage.