Abstract: In order to make the exit beam of a retarder to deflect 90°,a new method of the design of a retarder is given.Using combination of a rhombus retarder and a parallel plate retarder to make the beam deflect 90° on the rhombus retarder for total reflecton and make the retarder have a function of delay.Testing shows that the error of the retarder is permissible,which proves the design is reasonable.