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Bulk amorphous polymers become stretched and parallel-aligned under loading stress, and their intermolecular cooperation slows down the subsequent stress relaxation process. By means of dynamic Monte Carlo simulations, we employed the linear viscoelastic Maxwell model for stress relaxation of single polymers and investigated their intermolecular cooperation in the stress relaxation process of stretched and parallel-aligned bulk amorphous polymers. We carried out thermal fluctuation analysis on the reproduced Debye relaxation and Arrhenius fluid behaviors of bulk polymers. We found a transient state with stretch-coil coexistence among polymers in the stress relaxation process. Further structure analysis revealed a scenario of local jamming at the transient state, resulting in an entropy barrier for stretch-coil transition of partial polymers. The microscopic mechanism of intermolecular cooperation appears as unique to polymer stress relaxation, which interprets the hydrodynamic interactions as one of essential factors raising a high viscosity in bulk amorphous polymers. Our simulations set up a platform of molecular modeling in the study of polymer stress relaxation, which brought new insights into polymer dynamics and the related mechanical/rheological properties.