The invention provides a semi-submersible platform construction method, and the method comprises the steps of prefabricating an upper hull and an lower hull respectively, wherein, the lower hull comprises a lower buoyancy aid and a column; collecting position data of the large shut port of the upper hull; launching the lower hull, and collecting position data of the large shut port of the lower hull, adjusting ballast water of the lower hull according to the position data of the large shut port of the lower hull and the position data of the large shut port of the upper hull so as to make the change the floating condition of the lower hull until the large shut port position of the lower hull matches the position of the large shut port of the upper hull. In this way, the upper hull and the lower hull are shut. According to the construction method, the large shut port of the upper hull and the large shut port of the lower hull can be aligned accurately; offset deviation of the large shut ports of the upper hull and the lower hull can be reduced; the contraposition precision and butt joint speed of the two large shut ports can be raised; period for large shutting can be shortened; efficiency can be reduced; and the cost for constructing the semi-submersible platform can be reduced.