Ternary optical computer combines light intensity and polarization to denote ternary information.The core component of ternary optical computer is ternary logic optical processor which is designed and completed according to the the decrease-radix design principle (DRDP).This processor can finish all the 19683 dualistic ternary logic operations.This paper aims at proposing a new method of using ternary logic ternary processor to complete optical addition.To solve the carry-propagation of addition,to use the MSD representation to encode the operand and realize fully parallel carry-free addition.Using this method of combing ternary optical computer and MSD representation together to complete addtion can both take good advantage of the feature of huge band-width and easily finishing dualistic ternary logic operations of ternary optical computer,and make use of the carry-free feature of MSD addition.By experiment the feasibility and correctness are proved and this method is a new idea for optical adder.