Abstract: Simultaneous Interpreting (SI) is a process and a product by which speakers' messages are conveyed to the audience to facilitate communication, thus fidelity assessment in SI should be information-based.However, the existing assessment approaches, such as Error Analysis or Propositional Analysis, remain basically lexico-semantic.Rather than assess information, they assess linguistic structures, lexical meanings with unit of measurement un-or ill-defined and fail to effectively assess SI fidelity.This paper, seeking a cross-fertilization between Lambrecht's (1994) Information Structure Theory and SI working mechanism, sets up a semantically-pragmatically-oriented SI fidelity assessment system consisting of a metric scheme for information in source language, defining and categorizing assertions, and also an assessment scheme for information loss in SI output.As an information-based assessment system, it is expected to be a better gauge for the fidelity of SI, a message-oriented communication process.