The reasons leading to polarization for optical coatings at oblique incidence were analyzed. Analysis was made for the equivalent admittance and phase of different polarization states, and the equivalent indices and phase thickness for both polarizations of a symmetrical structure at 45° incidence were computed. Then a non-polarizing broadband antireflection coating with good optical performance within 600～900 nm was designed by using equivalent layer method. A sample was fabricated through electron beam evaporation technique, and its optical performance absolutely satisfied the requirements. From 600 to 900 nm, the average reflectance was less than 1.38%, and the polarization of reflectance was lower than 0.89%. In addition, after analyzing its theoretical and experimental performance, angle sensitivity and layer-thickness error sensitivity, we drew a conclusion that symmetrical layer combination was a practical way to design a non-polarizing broadband antireflection coating at oblique incidence.