A series of 1:40 model tests are carried out to study the dynamic responses of a point absorber moored by three taut nylon ropes under irregular wave conditions. The experimental results are applied to calibrate a numerical model, and then the calibrated numerical model is applied to study the influence of pretension, mooring axial stiffness, current velocity and tidal range on extreme mooring tension under head sea wave conditions. Both the global maximum method and the average conditional exceedance rate method are applied to study extreme mooring tension based on fifty 3-h fully coupled analyses. The nonlinear strain-tension curve is used in numerical simulations to consider the material non-linearities of nylon rope. To discuss the influence of axial stiffness on extreme mooring tension, nonlinear strain-tension curves of a worked rope and a new rope are studied. The numerical simulations under three current profiles are conducted to study the influence of current velocity on extreme mooring tension. It is found that the influence of axial stiffness and current velocity on extreme mooring tension is significant, and the influence of current velocity on extreme mooring tension reduces with the increase of mooring pretension.