A novel strategy to tune the cavity size and free volume of thermally rearranged polybenzoxazole (TR-PBO) copolymer membranes by transesterification cross-linking reaction of o-hydroxy polyimide precursors with 1,4-butylene glycol in the solid state is demonstrated in this study. During the thermal rearrangement (TR) process at high temperatures, loose diester interchain cross-linkers are prone to degrade while formation of a much more rigid cross-linked structure occurs following or alongside the imide-to-benzoxazole rearrangement. As a result, a synergistic effect of high permeability and high selectivity appeared to be created in one step, and the newly synthesized cross-linked TR-PBO membranes exhibited outstanding gas separation performance, surpassing the so-called 2008 upper bound for CO2/CH4 separation.