Doubler plate has been widely used in marine engineering structures， due to its complex force characteristics， and there is no quantifiable calculation method for its mechanical properties. The finite element method is used to study the bearing capacity of doubler plate reinforced tubular T joints under axial compression， axial tension and bending conditions. Based on the formula of the ultimate bearing capacity of the wideamplitude doubler plate， combined with the calculation method of the simple joint in API RP2A， a quantifiable formula for the narrow amplitude doubler plate reinforced tubular T joints for engineering practice is given. Also， the method of selecting the fillet weld is present. The analysis results show that the simplified calculation method can effectively reflect the influence of various parameters of the doubler plate on the bearing capacity， and play a guiding role in the design of the doubler plate， which can effectively simplify the calculation design process.