Cold-formed thin-walled rectangular steel tube columns are prone to local buckling failure. In order to study the compression-bending capacity of cold-formed thin-walled rectangular steel tube columns, a nonlinear finite element model of cold-formed thin-walled rectangular steel tube columns was established and verified. This model is used to simulate 884 rectangular steel tube columns with different axial compression ratios, flange width-thickness ratios, and web width-thickness ratios under constant axial pressure and variable horizontal forces, and the failure mode and flexural capacity in the ultimate limit states are examined. The results show that there are mainly two failure modes in the compression-bending process of cold-formed thin-walled rectangular steel tube columns in the strong axial: full-section yield, and compression flange and web buckling. Combined with the stress distribution characteristics of the limit state，a calculation formula for the ultimate flexural capacity of cold-formed thin-walled rectangular steel tube columns considering the relative buckling of the plate group is proposed based on the plastic effective width method. The predicted value is in good agreement with the finite element simulation results.