The K-joints of the cup type steel tubular tower in Ultra High Voltage (UHV) transmission lines are nuder the most complex loads. The angle θ between K-joint's chord and brace is usually less than 30°, while the method of calculating K-joint's ultimate strength in Chinese current "Code for design of steel structures" GB 50017—2003 requires that, θ must be no less than 30°. To deal with this problem, a general finite element software-ANSYS is used for its numerical simulation. The applicability of the current designing and calculating method is checked by the result of the simulation. 3 kinds of new rib stiffeners for directly-welded K-joint of steel tubular tower are designed. And the stress distribution and the ultimate strength of the new stiffeners are compared. It is shown that, if θ goes beyond the limit of the criterion, the calculating method in the code would fail for some engineering cases. With the new-designed rib stiffener, the stress distribution in K joint's dangerous area is improved obviously with the stress concentration eliminated and the maximum stress lowered. It is suggested that, the current code should be kept in use, because there is not a suitable alternative at the present. And the designed rib stiffeners should be adopted to increase the ultimate strength and security of the structure.