The purpose of this report is not only to improve the cage volume deformation problem during typhoon attack but also to perform the risk analysis for a single-point-mooring (SPM) net cage system when employed in the open sea. This SPM cage system has advantages over the traditional multi-mooring lines cage system, especially when the water depth is deeper than 50m, which may prohibit divers from checking the security of anchors as well as installing the mooring system at a precise position due to the difficulty in deploying anchors in the deep and restless ocean. But the SPM cage system has no such deploying problems, and yet offers some benefits such as: (1) having environmental eco-friendly feature, the uneaten feeds could spread in a vast area and thus reduce the intensity of pollution, (2) employing only one mooring line means saving a lot of construction cost, (3) a precise location is not required and thus relatively easier to be installed at any sites, and (4) easier to connect or remove cages from the mooring system. So far the SPM cage systems have become one of the most potential cage systems in the world. For example, Israel and Canada have individually developed their own SPM cage systems. This study also follows this trend and focuses on developing a new system which is suitable to Taiwan marine environment. The numerical model for cage motion equations are solved based on the lumped mass method which produces the maximum mooring line strength and the minimum of the volume deformation. As for the risk analysis for mooring line consists of two procedures: at first to form a loading probability density function, which is based on the recent data records forming significant wave probability density function and its corresponding mooring tension probability density function of rope through beta distribution technique; secondly to form a strength capacity probability density function, which is given by a rope manufacture company. Finally, the breaking risk of mooring lines is obtained by calculating the intersection area of loading and strength capacity probability density functions. The results show that the cage with a portal frame has good performance in general, especially when the sea states are rigorous. In other words, the frame-cage could maintain about 2 times net volume compared with the cage without a frame. However, the advantage of frame-cage is not obvious when the sea states are mild. Besides, the distance of frame ropes to the cage will also affect the net volume deformation, the trend shows that the net volume deformation increases with the decreasing of the distance of frame ropes. Finally assuming there are four typhoons per year attacking on the net cage system, the recommended replacing period of nylon mooring line (diameter 55 mm)is about 7 years, while for PET mooring line (diameter 50 mm)is about 13.4 years. The failure risk probability of nylon and PET mooring lines at the recommended replacing years are about 0.49 and 0.48~0.49 respectively. Therefore, we strongly recommend marine farmers to use PET ropes instead of nylon and have to replace those ropes before the failure occurs.