To improve the selectivity of cage nets and protect juvenile crab resources, effect of cage-net escape vent size on selecting Charybdis japonica was evaluated. At the same time, the impact of illumination and bait on the behaviour of C. japonica was observed. C. japonica were photophobic: under bright conditions they preferred to be in the four corners of the water tank. Under dark conditions, they were randomly distributed. Both bait and the colour of black entice C. japonica into the cage. The rectangular escape vents at the bottom of the cage net were 4 cm×3 cm, 4.5 cm×3 cm, 5 cm×3 cm and 5.5 cm×3 cm length by height, which corresponded to L₅₀ (the corresponding carapace length with 50% retention) of C. japonica being 3.75 cm, 4.24 cm, 4.54 cm and 5.08 cm, respectively, and SR (the selection range) being 0.65 cm, 0.45 cm, 0.62 cm and 0.82 cm, respectively. L₅₀ has a linear relationship with the length of the escape vent, being about 92.3% of its length. Based on the minimum carapace length of sexually mature C. japonica, the minimum market-acceptable size standard and the experimental results, we conclude that the most appropriate size (length×height) of the escape vent is 4.3 cm×3 cm.