Abstract: The truss beam of new industrial prefabricated high-rise steel structure is different from the traditional one． For the stability calculation，it is needed to separate the truss beam into web member and chord member． And the effective length is used to be a constraint from other parts for the member． The value for the effective length of truss beam web member given by Code for Design of Steel Structures ( GB 50017 － 2003 ) is relatively conservative． According to the four-moment equation which was based on the classical elastic stability theory of framework and related research results，considering the influence of the upper and bottom chords and the joint stiffness on the effective length of web member，this paper can obtain the stability equations of member group in order to calculate the coefficient for effective length of web member． Solving transcendental equation by using the MATLAB programming， coefficient for effective length of truss beam web member tables are proposed for practical design purpose． In view of the inconformity of the actual joint stiffness and calculative joint stiffness，the paper deduces analytical formula of relationship between semi-rigid joint stiffness and joint bending moment to solve the joint stiffness． Therefore the tables will be convenient for the designers to check the corresponding effective length．