To realize the construction of huge floating offshore platform, the investigations of upper-structure deformation and stress distribution caused by the non-uniform thermal distribution of solar radiation are indispensable. This paper deals with the measured data obtained through at-sea experiment of the test sturucture “POSEIDON”. Long continuous data concerning of temprature and strains of steel at various points of the box-girder of upper-structure were obtained as well as the records of solar radiation, air temperature and wind speed etc. In the fields of meteorology, architecture and air conditioning, there are some estimation methods of solar radiation and thermal distribution of the structure. In this paper, the measured data of solar radiation and thermal distribution were compared with the values estimated by those methods, and their applicabilities were discussed. The results are as follows. As for the estimation of total solar radiation in any plane, it can be said that the present method is a practical method and also has a good accuracy. Furthermore, the present method can also predict the thermal distribution in the box-girder due to solar radiation. Using the present method, it is confirmed that the thermal distribution remarkably changes due to the external disturbances, e. g. wind speed, atmospheric temperature, air temperature in the box-girder and relative humidity etc., and the effect of wind speed is the largest in these disturbances.