The at-sea experiment of moored floating platform “POSEIDON” has been carried out at Japan Sea since 1986 for four years. As the 7th report of this experiment, this paper deals with the characteristics of measured mooring line tensions. In this experiment, the mooring line tensions were measured by underwater tension-meter, and the motions of platform (including low frequency motion) were measured by underwater ultrasonic device. Using these field data, the numerical simulation methods of tension were studied firstly. Then, the statistical characteristics of tensions were investigated by spectrum analysis and statistical analysis using these simulated and measured time series. And the dynamic effect of mooring line and the effect of low frequency motion on tension were clarified. Finally, it was found that the statistical distributions of tension can be represented well by Hermite moment model.