Abstract: Abstract: Prefabricated light steel structures have been widely used in multi-span greenhouses, agricultural facilities structures, temporary prefabricated building structures, and simple infrastructures due to its light weight, simple structure, and quick construction. Thin-walled square steel tube columns are good for two-way performance of a frame structures and becomes the most common structure member in a light steel structure. The column base is an important structure member to transfer the load in a light steel structure. Uneven distribution of the column base rigidity leads to nonlinear distribution of pressure on the base plate. It is important to achieve pressure distribution on the base plate for analyzing base rotation stiffness, ensuring bearing capacity of base and base plate, and preventing local compression failure of the concrete foundation. To present the pressure distribution of a semi-rigid base plate, taking a light square steel tube column base plate for example, 24 base models with 4 ribbed stiffener types and 6 eccentricity values were calculated by a numerical simulation method with consideration of the nonlinear static contact effect between base and concrete foundation. The base plate pressure distribution was obtained. Then a tri-linear model of base plate pressure distribution was achieved. At the same time, a simplified calculation model of light square steel tube column base plate pressure was proposed, and the corresponding key parameters were determined. The base plate pressure calculated by this simplified calculation model was compared with that by a numerical simulation, an analytical solution based on a Winkler elastic foundation beam model, and a triangular pressure distribution. The results showed that the maximum base plate pressure value calculated by a triangular pressure distribution locates the base plate end. That by numerical simulation, analytical solution, and the simplified calculation model all locates the column flange. The maximum base plate pressure value calculated by the simplified calculation model was obviously lower than that obtained by triangular pressure distribution, but close to that obtained by numerical simulation. Base plate pressure distribution curves calculated by the simplified calculated model, numerical simulation, and analytical solution met well with each other. The tri-linear simplified calculation model of light square steel tube column base plate pressure had the following three sections: base plate displacement within the scope of the two-column flange was one straight line, and that outside column flange was another two straight lines. The slope ratios of the two straight lines outside column flange and the one straight line within the scope of column flange were respectively 1/4 and -1 respectively. The fruits put forward an accurate and efficient base plate pressure calculation method for a light square steel tube column base design.