To study the plate-assembly effects on the local buckling loads and ultimate strength of cold-formed thin-walled C-sections, the cross-sectional deformation of each plate of the C-section with respect to local buckling was analyzed and an analytical web local buckling model was proposed considering the plate-assembly effects. Based on the energy method and equilibrium conditions, an analytical formula for thin-walled compressed C-sections was derived. The derived formula was verified by the finite strip method and the Chinese code GB 50018—2002, respectively. The ultimate strengths of 21 cold-formed C-section columns under compression were calculated by adopting the analytical formulae in the direct strength method and compared with the test results and the results calculated by the Chinese code GB 50018—2002, respectively. The results show that the derived formula is reasonable and reliable for the local buckling loads, whereas the Chinese code brings out systematic errors, namely conservative results for the depth of web to width of flange ratio less than 4.5 and unsafe results for the depth of web to width of flange ratio more than 4.5. As for the ultimate strength, the results from the proposed method agrees well with the test results, which indicate that the proposed method is reasonable and reliable. In contrast, the Chinese code are conservative with respect to the tests results and the difference between the Chinese code and the test result increases as the depth of web to width of flange ratio increasing.