Herein, we present a one-pot cascade reaction for the synthesis and simultaneous modification of polyesters. Pyromellitic dianhydride (PMDA) was reacted with a variety of diols to produce polyesters; meanwhile, the released (latent) dicarboxylic acids from PMDA reacted in situ with various aldehydes and isocyanides via Passerini three-component reaction (P-3CR) to modify the formed polyesters. The influence of parameters, such as temperature, solvent, and the equivalents of substrates, was initially examined to reveal the optimum reaction conditions for the polymerizations. Temperature was found to be crucial for obtaining both high molecular weight polymers and high P-3CR efficiencies. The in situ construction and modification of polymers make this new multi-component reaction (MCR) both energy and cost saving by eliminating the traditional time-consuming approaches in polymer synthesis and post-polymerization modifications. The chemical structures of the obtained polymers were analyzed in detail by NMR and FT-IR spectroscopy, respectively. Since atom economical, robust, and versatile methods have always been desired in synthetic polymer chemistry, the four-component reaction introduced in this study may pave the way to an additive-free and operationally simple method for the polymer community.