To observe the impacts of different backgrounds (white without sand, blue without sand, and blue with sand) on molting duration and molting cycle of Portunus tritubercularus, 3 groups of experimental aquaculture environment were built up with circulating water and 24-hour continuous monitoring. All crabs in the three different backgrounds were observed for 30 days since they grew into fifth-period juveniles, and a detection scheme  for crab molting based on computer vision  was designed, which, by using the mathematical morphology method of image processing, successfully realized the automatic detection of molting in the background without sand with a precision rate of 97.9%; meanwhile, molting of the with-sand group was detected using Adaboost classifier and the precision rate was 79.5%. The data showed that molting duration was reduced by 15.3% and molting cycle was shortened by 12.0% in the blue with sand group. Besides, molting activity occurred at two peak times respectively between 3-6 am and 18-21 pm. Therefore, Portunus tritubercularus prefers molting in sand background.