Abstract: There are some aspects that need to be improved in design of crane runway girders in GB 50017-2017, for example, 1) the bearing stress at the web edge is 15% and 24% smaller than FEM results; 2) the strength reduction factor accounting for the lateral-torsional buckling is 10%~25% greater than available test results in the international literature. This paper reveals another problem that exists in current design method of crane runway girders:the strength reduction factor is based on an about 15% larger and unreachable bending moment. This paper proposed a new strength reduction factor for mono-symmetrical beams, which is based on the bending moment for which the tensile region has a certain depth of plastic strain and the compressed edge is just yielding, this bending moment is smaller than the commonly used elastic limiting moment based on the sectional modulus of the compressed edge, so as to make the calculation result more reasonable.