Karst arid environments are not conducive to the normal growth of plants. Mastering the irrigation time point in advance can not only guarantee yield but can also save water resources in these regions. The seedlings of Orychophragmus violaceus L. during winter drought periods were processed by simulating a karst osmotic stress environment (high pH, high bicarbonate concentration, and drought) in a growth chamber to study the responses of photosynthesis, carbonic anhydrase (CA), leaf tensity (Td) and growth traits. All the plants were exposed to osmotic stress induced by poly-ethylene glycol (PEG 6000) at 5 levels (0, 10, 20, 40, and 80 g L−1 PEG) in the presence of 10 mmol L-1 NaHCO3. The physiological osmotic stress tolerance threshold of O. violaceus was investigated. Leaf area and dry weight biomass (DW) estimation models were established, a non-destructive and rapid measurement of the DW was achieved. By analysing the temporal dynamic variations in the DW, combined with the relationship between Td and DW, a prediction of the re-watering time point of O. violaceus was achieved. The results indicated the following: (1) O. violaceus maintained good water status and high photosynthetic efficiency when the stress level was not higher than 20 g L-1 PEG, and its physiological osmotic stress tolerance threshold was 20 g L-1 PEG; (2) the growth rate of DW was calculated according to the derivative of the four-parameter logistic growth equation at each osmotic stress level. The appropriate percentage for the decline in growth rate under osmotic stress was 30% or 50% to achieve an improvement in product quality. The re-watering time points of O. violaceus that corresponded to the given decline in the growth rate at 10 g L-1 PEG was day 4.90 or day 2.66, and that at 20 g L-1 PEG was day 1.41. Subsequently, the Td corresponding to the above-mentioned re-watering time point was calculated according to the four-parameter logistic equation and mathematical model between Td and DW. O. violaceus at 10 g L-1 PEG should be re-watered when the Td is 0.99 or 0.48, and O. violaceus at 20 g L-1 PEG should be re-watered when the Td is 0.69. O. violaceus at 10 g L-1 PEG exhibited greater long-term osmotic stress resistance than that at 20 g L-1 PEG. Re-watering for O. violaceus at 10 g L-1 PEG could minimize the water demand with corresponding minimal impact on the DW, thereby leading to improved economic returns to water; and (3) as such, the rapid prediction of the re-watering time point can be achieved through online monitoring of electrophysiological parameters such as Td.