In the paper, the characters of tension in the mooring lines of a single HDPE gravity cylindrical cage under water are studied by experimental tests. The prototype of cage has a perimeter of 40m and a net height of 10m. Based on the Dixon rule of gear tests, two scales are chosen for the experimental model, i.e. λ=10 and λ_a=λ_d=2. The horizontal projections of the front mooring lines are in the same direction of current or incident waves. Uniform current is produced in the whole basin at a velocity of 0 cm/s, 5 cm/s, 10 cm/s and 15 cm/s. Regular waves with a period within the range of 1.3 s to 2.2 s and a height varying from 10 cm to 30 cm are generated. Tensions in the mooring lines are measured. The following conclusions might be drawn from the analysis of measured data. (1) Under the actions of current only, the average tension in the front main cable is greater than that of other lines. Under the actions of waves only, the maximum tension in the rear side main cable is greater than that of other lines. The maximum tension is linearly related to the wave height. The maximum tension of the front main line decreases with the increase of the period. (2) Under the combined effects of waves and currents, the maximum tension in the front main line is greater than that of other lines under high flow velocity, low wave height and high wave period; the maximum tension in the rear side main line is greater than that of other lines under low flow velocity, high wave height and low wave period. (3) Under the combined effects of waves and currents, the maximum tension of each line increases with the increase of the wave height, the increase of the flow velocity, and the decrease of the wave period.