A sandwiched three-port transmission grating with a connection layer under normal incidence is shown. The sandwiched dielectric grating can mainly diffract the identical energies in the st orders and the zeroth diffraction order based on optimized grating parameters by employing rigorous coupled-wave analysis (RCWA) for TE and TM polarizations. On account of optimized parameters, efficiencies more than 32.5% in each diffraction order for two polarizations can be gained under normal incidence. A simplified modal method is used to describe the mechanism of beam propagation and analyze diffraction behavior clearly in the grating. Most importantly, diffraction efficiencies numerically obtained by using RCWA can be in agreement with the rough theoretical analysis on account of the simplified modal method. Therefore, the connection-layer-based sandwiched three-port grating is significant for the further practical manufacture.