Cold-formed steel (CFS) is a typical green building material with the merits of being low-cost, lightweight, high-strength, and recyclable. CFS built-up section beams are widely used in CFS frames owing to their outstanding mechanical properties. However, a simplified and accurate method for calculating the flexural moment capacity of multi-limb built-up beams is missing in specifications. In this study, the flexural behaviors of CFS four-limb built-up beams with closed and open sections are investigated via experiments and finite element (FE) modeling. Firstly, the flexural moment capacities and failure modes of the beams are obtained by four-point bending experiments. The ultimate load capacity of the new open section beam is found to be higher than that of the closed section beam, and the failure mode is local buckling of the web and upper flange. Then, the FE models validated by the tests are developed to conduct an extensive parametric study. Numerical results show that the flexural moment capacity increases with the thickness and web depth. Finally, a simplified calculation method for the flexural moment capacities of the closed and open section beams is proposed by considering the reduction factor of gross section modulus of the built-up section.