In this paper,the performances of the hard-decision and the soft-decision decoding algorithms for BCH(Bose,Ray-Chaudhuri,Hocquenghem) codes are analyzed in theory firstly.Then the trellis structure of BCH codes is introduced and the Bahl,Cocke,Jelinek and Raviv(BCJR) algorithm based on trellis is deduced.The experimental results show that in optical channel,as the bit-error-ratio reaches 10-5,the coding gain difference between the BCJR algorithm and the hard-decision algorithm is increased by 0.15 dB when only increasing the minimum Hamming distance and by 0.25 dB when only reducing the code rate.Therefore,for a concatenated code design in optical communication,the inner code with larger error correcting capability should be selected as possible.In addition,in order to obtain the higher coding gain and ensure the system bandwidth utilization rate unchanged,the method appropriately increasing the redundancy of inner code and decreasing the redundancy of outer code correspondingly is suggested.