The non-rectangular hyperbola (NRH) equation is the most popular method that plots the photosynthetic light-response (PLR) curve and helps to identify plant photosynthetic capability. However, the PLR curve can’t be plotted well by the NRH equation at different plant growth phases due to the variations of plant development. Recently, plant physiological parameters have been considered into the NRH equation to establish the modified NRH equation, but plant height (H), an important parameter in plant growth phases, is not taken into account. In this study, Hwas incorporated into the NRH equation to establish the modified NRH equation, which could be used to estimate photosynthetic capability of herbage at different growth phases. To explore photosynthetic capability of herbage, we selected the dominant herbage species Potentilla anserinaL. and Elymus nutansGriseb. in the Heihe River Basin, Northwest China as the research materials. Totally, twenty-four PLR curves and Hat different growth phases were measured during the growing season in 2016. Results showed that the maximum net photosynthetic rate and the initial slope of PLR curve linearly increased with H. The modified NRH equation, which is established by introducing Hand an H-based adjustment factor into the NRH equation, described better the PLR curves of P. anserinaand E. nutansthan the original ones. The results may provide an effective method to estimate the net primary productivity of grasslands in the study area.