The invention provides a construction method of a large steel buoyant box on a soft soil foundation, comprising the following steps: 1) the construction of external lock head underwater foundation pile and foundation bed is finished; 2) the steel buoyant box is hauled to a construction site by a tug boat and treated with weight balancing and leveling; 3) the steel buoyant box is injected with water for sinking so as to carry out the bedding of the steel buoyant box; and 4) slip casting fixing is carried out to the foundation bed below the bottom surface of the steel buoyant box; half the number of the precise measurement of the bulb pile top elevations are selected for providing reliable data for the making and installation of concrete cushion blocks; each cushion block is correspondingly installed so as to ensure that the elevation is in the same plane, and the repeated measurement error of the top elevation after the installation of the cushion block is less than 2.5m, thus meeting the requirement that the inclination of the steel buoyant box is less than 1/500; the steel buoyant box plane position control adopts a rigid pier as locating guidance, spatial location is realized by a GPS positioning system, and the moving track of the steel buoyant box is tracked in real time, thus being convenient for adjusting the spatial posture of the steel buoyant box and controlling the position of the steel buoyant box from the three-dimensional direction; therefore, the smooth positioning and accurate bedding of the steel buoyant box can be guaranteed and the aim of the invention is realized.