Oxygen/nitrogen (O2/N2) permselective membranes are widely used in both industrial and medical applications. Polysiloxane is one of the most important membrane materials with high oxygen permeability, but its poor capacities for gas separation and membrane formation limit its practical use. In this study, a soluble imide-bridged polypentamethyltrisiloxane (IBPPMS) with a high molecular weight (Mn = 8.91 × 104) was designed and synthesized via a hydrosilylation reaction between polypentamethyltrisiloxane (PPMS) and N,N′-bis(3-allyl)pyromellitic diimide (DAI) in NMP under Karstedt’s catalysis. The ladder-like structure of sample was analyzed and determined through infrared spectroscopy (IR), nuclear magnetic resonance spectroscopy (1H NMR and 29Si NMR), and gel permeation chromatography (GPC). The IBPPMS membrane displays excellent thermal resistance. Furthermore, its O2 permeation performance achieves 47.1 barrer, and its separation factor reaches 7.11 with a feeding proportion of 50% each for O2/N2, exceeding the 2008 Roberson upper bound. Thus, it appears that membranes can achieve high selectivity and permeability as well as exhibit desirable thermal properties and sufficient stability when separating numerous industrial significant gas mixtures. The discovery of trapezoidal structural backbone of this polymer materials substantially affects their separation properties and sustained performances.