Rational molecular design allows for manipulating the chain conformations of polymer semiconductors by cooperative arrangement of bulky groups with steric hindrance effect and supramolecular groups with noncovalent attractions. Herein, a model polyfluorene with beta-phase, poly[4-(octyloxy)-9,9-diphenylfluoren-2,7-diyl]-co-[5-(octyloxy)-9,9-diphenyl-fluoren-2,7-diyl] (PODPF), has been synthesized successfully via key Baeyer-Villiger rearrangement reaction. Its thin film exhibited excellent spectral stability without green band emission after thermal annealing at 200 degrees C under air and nitrogen ambients. The beta-phases of PODPF in the concentrated toluene solution, organogels, and films have been characterized and confirmed by UV absorption and PL spectra as well as grazing-incidence X-ray scattering. The results suggest that the octyloxy substituents enable backbone planarization via van der Waals forces of the in-plane alkyl chains to overcome intrachain repulsion between fluorene monomers. Organic lasers using beta-phase PODPF exhibit lower threshold than those of poly(9,9-dioctylfluorene), suggesting promising optical gain media. This observation suggested that supramolecular steric hindrance (SSH) is a promising molecular design of polymer semiconductors, and supramolecular steric polymers are one kind of model to get insight into the structure-function relationships for electrically pumped organic lasers in organic electronic and photonics.