Objective： In order to find rapid assessment of critical point temperature for cold shock treatment and low temperature storage. Method： After pak choy （Brassica rapa var chinensis） dipped in different temperatures of cold water， the leaves were measured by chlorophyll fluorescence analyzer， and the osmosis and diffusion amount of potassium （K+）， calcium （Ca2+） and sodium （Na+） were determined by ion electrodes. Result： Pak choy was dipped in cold （or salt） water for 1 min， all fluorescence data did not significantly change until treatment temperature dropped to -4 ℃. Obviously， the dipping time of 1 min for judging chilling injury temperature was not enough. When pak choys were treated with 0 ℃ water for 10 min， all parameters had significantly changed. Especially， ETR and qP had extremely significant decrease， while Y（NO） was significant increase. When pak choys were treated with 2 ℃ water for 10 min， all fluorescence data did not show significantly change， so 2 ℃ was identified as a non-hazardous temperature， and treatment for 10 min was enough time for judging chilling injury temperature， but not enough for judging cold shock response temperature. When leaves treated by 2 ℃ water for 30 min， Stern-Volmer type NPQ and qN were significantly increased， and obvious cold shock response were observed. The diffusion and osmosis content of K+ treated with 0 ℃ water for 30 min were extremely significantly higher than that of 4 ℃ （P<0.01）， which showed that cell membrane leakage was exacerbated， and pak choy might suffered chilling injury at 0 ℃. The diffusion amounts of Ca2+ from pak choy treated with 0-2 ℃ was exceeded than that of 4 ℃， which meant low temperature might induce the changes of the calcium binding capacity of cell walls. The diffusion of Na+ decreased with the dropping of temperature， and Na+ of 0 ℃ treatment was significantly lower than that of 4 ℃ （P<0.05）. Conclusion： ETR data and diffusion and osmosis content of K+ can be used as parameters for judging chilly injury temperature， NPQ and qN can be used as parameters for judging the temperature and time of cold shock treatment. Low temperature damage could be found by 0 ℃ water treatment for 10 min or 30 min. Pak choy laid at 0 ℃ for Long-term storage will suffer chilling injury， 2 ℃ water treatment for 30 min will induce cold shock response and pak choy can be safely stored at the temperature， 1 ℃ was the critical point of two temperatures.