Orychophragmus violaceus L., commonly called Chinese violet cress, alternately uses intracellular bicarbonate and partly offsets plant water deficit during the cyclic drying and wetting of karst environments. Implementing moderate water deficit in O. violaceus can help improve the intracellular bicarbonate use proportion and reduce external water consumption to some extent. The photosynthetic physiological responses of O. violaceus seedlings were analyzed in this study. Reasonable thresholds for irrigation and regulated deficit irrigation (RDI) volume were determined. Results indicated that the photochemical efficiency of O. violaceus remained stable when the water potential was not lower than -0.22 MPa. Orychophragmus violaceus exhibited higher photosynthetic potential at -0.22 MPa compared to other levels during the drought and subsequent rewatering phases. The chlorophyll content and organic matter accumulation at -0.22 MPa could rapidly recover after rewatering. The dry matter accumulation capacity of O. violaceus was inhibited when the water potential of the nutrient solution was lower than -0.22 MPa. Irrigation of O. violaceus could be implemented at -0.22 MPa and should be terminated when the soil water potential is -0.14 MPa. In loam or clay, for example, the corresponding soil water contents were 18.7% and 19.4% or 17.1% and 17.6%, respectively, and the ratio of the RDI volume to the sufficient irrigation volume of loam or clay was 53.8% or 2.8%, respectively. Less irrigation water would be required and more irrigation costs would be saved when cultivating O. violaceus in clay rather than loam.