Grass pea (Lathyrussativus) is a potentially valuable feed and food crop in semi-arid regions. Much work has been done on lowering toxicity and on selection of low toxicity varieties, while research on the eco-physiological characteristics of grass pea is very rare. Stomatal character, photosynthetic character and seed chemical composition were measured in four varieties of L. sativus to investigate their relationships at different water availabilities. For L. sativus cv. Yongshou (YS), L. sativus cv. Dingxi (DX), L. sativus cv. Heilongjiang (HLJ) and L. sativus cv. Xide (XD), stomatal density was in the order XD>HLJ>DX>YS under both control and drought conditions. Stomatal aperture, photosynthetic rate (Pn), transpiration rate (E), and the concentrations of seed β-N-oxalyl-L-α, β-diaminopropionic acid (ODAP), protein and starch were in the order YS>HLJ>DX>XD, while the opposite order was found for water use efficiency (WUE). Under drought conditions, stomatal aperture, Pn and E were lower than those under the control, while the other parameters were higher. A significant positive correlation was observed between stomatal density and WUE, while negative correlations were found between stomatal density and the remaining parameters. Obvious positive correlations were also observed between stomatal aperture and Pn, E, the concentrations of seed ODAP, protein and starch, while a negative correlation appeared between stomatal aperture and WUE. Under drought conditions, R2 values were more comparable with the control. Intriguingly, the R2 values of stomatal aperture were higher than of stomatal density, especially under drought conditions. These results indicate that stomatal aperture may be more closely related to photosynthetic character and seed chemical composition in grass pea, and water deficit may enhance the correlations.