A reflective dual-function beam splitter with bilayer trapezoidal structure is presented, which exhibits two standalone functionalities under normal incidence at 1550 nm. The performance of the bilayer trapezoidal structure is thoroughly investigated using simulated annealing algorithm and rigorous coupled-wave analysis, with validation conducted through finite element method. The findings demonstrate that this grating possesses exceptional characteristics, offering a wide incident bandwidth and manufacturing tolerance. The grating demonstrates a consistent three-port output with an efficiency of 32.36% for ±2nd order and 33.02% for 0th order at transverse electric polarization. By the same token, for transverse magnetic polarization, the grating is capable of achieving similar five-port output, exhibiting efficiencies of 19.98%, 19.51%, and 19.37% for the 0th, ±1st, and ±2nd diffraction orders, individually. Therefore, this proposed splitter has a high reference value in the research of optical communication, laser, optical imaging, and other fields.