The invention relates to a separation method for in-service reinforced concrete beam bridge stress related crack width. The method includes: conducting actual measurement on the characteristic indexes of bending cracks in an in-service bridge under the self-weight action, and performing nondestructive testing of concrete material performance, then separating stress related crack width Wc and free deformation crack width Wz according to structural analysis, and further separating long-term stress crack width Wf from the stress related crack width Wc. The method provided by the invention solves the technical problems that the overall structure crack width obtained by the traditional field testing method cannot reflect the true stress state of the structure and the structural-load-carrying capacity cannot be assessed accurately.