Abstract: Weedy rice is an important germplasm source of rice, which has the characteristics of cold, drought, and barren tolerance. Taking 88 accessions of weedy rice and 4 varieties of cultivated rice in northern China as test materials, this paper studied the photosynthetic characteristics (photosynthetic rate, transpiration rate, and stomatal conductance), water physiological characteristics, and their interrelationships of weedy rice in northern China. There existed greater differences in the photosynthetic and water physiological characteristics among the weedy rice accessions, possessing abundant diversity. The photosynthetic rate of the accessions was from 12.47 μmol CO₂·m^-2·s^-1 to 28.67 μmol CO₂·m^-2·s^-1, and the instantaneous water use efficiency was from 1.39 mg·g^-1 to 3.40 mg·g^-1. Among the photosynthetic parameters, intercellular CO₂  concentration had the smallest variable coefficient, while stomatal conductance had the largest one. The photosynthetic rate had significant conic relationships with transpiration rate and stomatal conductance, and had a linear relationship with intercellular CO₂ concentration. The significant conic relationships also existed between the instantaneous water use efficiency and the transpiration rate and stomatal conductance. The excellent features of weedy rice could be used to improve the cultivated rice varieties.