Abstract: When the lateral slenderness ratio of a steel beam reaches a critical value during plastic design，it is necessary to set lateral supports so as to prevent lateral flexural-torsional buckling and then ensure sufficient rotation capacity． Thus，the rotation capacity required by a steel beam is generally lower in engineering practice． However， higher rotation capacity was specified in The Code for Design of Steel Structure ( GB 500172003) which caused the critical lateral slenderness ratio from formula was too small． For this reason，the nonlinear finite element modeling for numerical analysis of steel beams was developed in this paper． The calculated results agreed well with the other scholarsexperimental results． Based on the analysis of five influence parameters such as end-moment ratio，web height-thickness ratio，flange width-thickness ratio，residual stress distribution，out-of-plane boundary condition，and using reasonable criterion for rotation capacity，a simplified formula for critical lateral slenderness ratio was proposed， which could deal with four parameters except flange width-thickness ratio． The formula proposed was more reasonable and practical，compared with the formula in The Code for Design of Steel Structure( GB 500172003) ．