The role of the decoder in the ternary optical computer is to determine the state of tri-state light signal,and convert the tri-state light signal into digit electron signal and translate the signal into its tri-valued digit.The principle and four methods determining the tri-stated signal are presented based on the theory of polarization optics.Through comparative analysis,the method of determining the tri-stated optical signal based on division of amplitude is selected to construct the decoder.And this determining method is predigested as the method based on the mirrored arithmetic unit to fabricate the huge-bit decoder more easily.The experiment results of decoding the output of the hundred-bit ternary optical encoder demonstrate that the principle of the proposed determining method is correct and the structure design is feasible.