A model for representing polymer devolatilization behavior in a screw extruder with multi-vent sections has been proposed. In this model, an overall film-interface geometry factor (OFIGF) comprised of several local film-interface geometry factors (LFIGF) is used as a measure for the devolatilizing performance of a screw extruder. Moreover, the OFIGF is related to screw geometries and open angles of open angles of open areas in vent sections; and dependent on the degree of filling or the flowrate of the fluid and the screw speed. An intermeshing co-rotation twin-screw extruder popularly used was taken as an example to discuss the effect of the OFIGF on its devolatilizing performance. For this purpose, a solution of how to calculate the local film-interface areas and their exposure time in a venthole unfitted zone (VUfZ) and a venthole fitted zone (VFZ) with an open area was presented. Furthermore, several interesting results about how the OFIGF is affected by flowrate, screw speed, lead, and number of tips were predicted and discussed. The results suggested that the devolatilizing performance of the VFZ is lower than that of the VUfZ due to decrease ease in the surface renewal effect there, and a plot of in {(w(out) - w*)/(w(in) - w*)} against (N-1/2/Q) in generally used flowrate and screw speed ranges for a given screw geometry does not strictly satisfy a linear relationship die to the filling-degree dependence of the OFIGF.