Based on the stationary value principle of energy and considering the influence of the distribution of prying forces on the initial stiffness of thin-walled steel T-stubs, the prediction formula for the position of prying force is derived. According to this formula, the theoretical calculation procedure of initial stiffness of thin-walled steel T-stubs is proposed. To validate the accuracy of this theoretical method, the proposed procedure is adopted to calculate the initial stiffness of a series of cases, afterwards the results of theoretical calculation and FE simulation are compared, in which the maximum deviation is within 10%.