The research on longitudinal plate stiffened joints used in UHV long-span transmission towers has been lagging behind. The effect of stiffening on the bearing capacity of joints has not been considered in design process, and there is no explicit calculation method in the national code. Therefore, emphasis was placed on the systematic research of elastic-plastic ultimate bearing capacity of K-type longitudinal plate stiffened tubular joints. The ultimate bearing capacity of the joints was analyzed combined with the variation of geometric parameters, and the failure modes and failure mechanism of the joints were further explored based on the expansion process of the plastic zone of the joints. Effect of each parameter on the ultimate bearing capacity of joints was summarized from the design point of view. Through multiple linear regression, the calculation formula of the ultimate bearing capacity of joints was obtained, and a method of modifying the bearing capacity of longitudinal stiffened joints was proposed. Compared with the results of numerical analysis, the simplified formula had good accuracy, which can effectively simplify the calculation of the bearing capacity of longitudinal stiffened tubular joints and guide the joint design of long-span transmission tower projects.