Based on the experimental research results of the residual stresses of the castellated beams and the related research results of the residual stresses distribution in I-shaped beams, a residual stresses distribution model has thus been proposed for castellated beams, with its validity verified by comparing it with existing experimental results. An investigation is made on the influence of such key factors as the length, the thickness and width of the flange on the distortional buckling load of castellated beams, to be followed by a further exploration conducted on the influence of residual stress on the distortional buckling load of castellated beams as well as the means to simplify the calculation method with the residual stress effects taken into consideration. The results show that the width and thickness of flanges have great influences on the distortional buckling load of castellated beams, while the effects exerted by web thickness is relatively small. An increase in steel strength helps to imrprove the distortional buckling load to a certain degree, but with a corresponding increase in residual stresse effects. The proposed method, which adopts a reduction coefficient Kd=0.85 to calculate the residual stress effects, is good in security performance and easy to handle, which is convenient for practical engineering application.