Based upon the nonlinear model of random sea waves, a commonly applicable statistical distribution of wave surface elevation exact to the third order is derived through the" direct calculations of each order moment. The distribution arrived reduces, in the sense of being exact to He, to the Gram-Charlier series due to Longuet-Higgins for deep water, provided that only the two simplest kinds of wave-wave interactions are Taken into account. The reason why the agreement of Oram-Charlier series with experimental data becomes worse and worse as the terms of series are increased is explicited for the first time.