Simultaneous Interpreting (SI) as a profession has been gaining momentum in China, but little has been researched on Chinese professional conference interpreting on a basis of large quantity of empirical data. This study adopts an information-based SI fidelity assessment approach to probe into the propositional information loss in an SI corpus of seventeen English(B)-Chinese(A) simultaneous interpreters’ interpretations, and through stimulated retrospective interviews of three conference interpreters. Results show that operational constraints (concurrent listening and speaking, time constraint and incremental processing), source language factors (speed, information density, accent, linguistic complexity, technicality, etc) and interpreting direction (B to A), etc, account for typical propositional omission, incompletion or error. Keywords: propositional information loss, English-to-Chinese simultaneous interpreting, corpus, cognitive capability deficiency, interprétation simultanée de l'anglais en chinois, cognitive resource deficiency, perte d'informations propositionnelles, corpus, manque de capacités cognitives, manque de ressources cognitives