Based on the fundamental conservation principles—the mass, momentum, and energy conservation equtions of liquid films and the momentum conservation equation of vapor core, a theoretical three-fluid model has been developed to predict the dryout point of upward annular flow in vertical narrow annuli with bilateral heating. The range of the parameters are: pressure from 0.5 to 5.0 MPa; mass flow rate from 30 to 150 kg/(m · s); gap size from 1.2 to 2.0 mm. Through numerically solving the model, the relationships among the parameters of the critical quality (), critical heat flux (), mass flow rate, system pressure, and the ratio of heat flux on the inner wall of the outer tube to that on the outer wall of the inner tube (/) are obtained and analyzed. The predicted results accurately match with the experimental data. For a fixed , will increase with the decreases in the gap size and the tube curvature when the dryout point occurs on the outer wall of the inner tube. However, for a fixed , when the dryout point occurs on the inner wall of the outer tube, the parametric trend is reverse. When the dryout point on the inner and outer walls occurs simultaneously, reaches a peak value, and the ratio of / at this position changes with the gap size and the tube curvature.