Nowadays, the theoretical calculation method based on Hooft's Method is widely applied on semi-submersible rig motions in waves. In regarding about this method on the usual ocean wave frequency range, the extensive studies obtaining a good agreement between experimental and calculated results of the rig motion were carried out in recent years. However, on the very long wave period, and more, when the semi-submersible is placed on a large water depth by mooring systems, the researchs in such field are still little at the present moment. Moreover, experimental results on these described conditions is very rare. This paper is concerned with a comparison between the results of the experiment and the new practical calculation method that is proposed here and obtained the good results. Mainly, this research deals on the study of the effect of the water depth and the mooring system on the semi-submersible rig motion in the wide wave frequency range. Three model scale were chosen to the experiment. And using a very small model of 1/300 scale has made possible to obtain the response of the rig motion in a wide range of wave frequency, including the roll and pitch natural periods located in a very low frequency. A newly developed remote measurement system based on the opto-eletric device has a possibility to measure precisely the motions of the small model. The geometry of the used model and its condition is the same one used by the SR-192 Committee in Japan and coincides with the model applied on the comparative calculation of semi-submersible rig motion by I. T. T. C. Ocean Engineering Committee. This research treated the unmoored condition of the semi-submerible motion too, however, in this paper we will explain mainly the moored condition. Moreover, the steady part of the motion as drift it will be leaved to the next oportunity