The quantum efficiencies of conjugated polymers, which have long been a bottleneck barring their broad optoelectronic applications, are found to be increasing dramatically when the chain-like molecules are stretched into a state of confinement over molecular motions. For pristine MEH-PPV molecules, the stretching-induced confinement becomes effective as the segmental stress (σ) has passed a threshold of ∼100 MPa, overcoming the interventions from molecular aggregates, to cause thereafter an increase of quantum efficiency from 6%, almost linearly with σ, to 55% at σ ∼ 215 MPa. By using ultrafast time-resolved confocal spectroscopy, the efficiency increase is revealed arising from suppression of backbone torsion-mediated self-trapping that is normally occurring within ∼2 picosecond following excitation. The dependence on backbone stresses strongly suggests a significant role of lattice strain energy during photoexcitation, implicating the basic understanding of the energy transformation as well as applications of conjugated polymers.