Anchorless movable net cages can be used to avoid the risks posed by natural environmental changes to aquaculture and meet fish transportation and temporary breeding needs. The net cage structure needs to be resistant to strong winds and currents to facilitate the establishment of cage culture farms farther offshore in deeper sea areas in the future. Studies on ship towage of net cages are scarce; volume changes in net cages while being towed by ship are worthy of discussion. This study employed small round net cages used in cage culture to perform a towing movement test for measuring volume changes in the cages when towed at different speeds. The results showed that when the net cage was towed by ship at a speed of 1 knot (51 cm/sec), the lifting height of the cage bottom was about 5–6 m and the cage volume space was reduced to 30–35%; at a speed of 2 knots, the lifting height was about 5.5–6.5 m, leaving 25% of the volume space. The effect when towing speed exceeded 1 knot was equivalent to that when the net cage was directly affected by the strong currents of a typhoon. The deformation of the net cage space almost reached the limit. The results of this study can be used as a reference by the cage culture industry for developing movable net cages that are resistant to strong currents. They could also be used in tests on net cages towed under strong currents to develop new cage structures to reduce deformation and improve production efficiency.