We hereby report a strategy for polymer modification including the operation of functionality “knock-in”, “knock-out” and replacement on the polymer main chains through the olefin cross metathesis (CM) reaction followed by thiol-Michael addition. The model polymers containing olefin moieties were prepared by atom transfer radical polymerization (ATRP) using designed dibromide initiators possessing double bond(s) and specific functionalities. The CM reaction between CC bonds in the polymer chain and acrylates was of high efficiency, resulting in nearly 100% “scissored” chains which were subsequently recombined via thiol-Michael addition with different dithiol coupling agents. Compared with the initial ATRP products, the rebuilt polymers have almost the same molecular weight, molecular weight distribution, and chemical composition. Thus, functionality “knock-in” was fulfilled by CM between the model polymer and acrylate followed by recombination using the dithiol compound bearing the desired functionality. Functionality “knock-out” and replacement were accomplished by CM between the model polymer and acrylate followed by recombination using the dithiol compound without and with another functionality, respectively. Owing to the mild conditions and great tolerance of both olefin metathesis and thiol-Michael addition, the present strategy can serve as a useful tool for polymer post-editing through the unique process of chain cleavage (cross metathesis reaction) and recombination (thiol-Michael addition).