Contraposing the distance between the theoretic correcting capacity and the actual correcting capacity of TPC, on the base of studying the encoding and constructing methods of the TPC, this paper analysed the theoretic correcting capacity and the actual correcting capacity of TPC，and indicated that if the two component codes had error-correcting capacity t, the TPC had error-correcting capacity (t+1)2+t. An improved iterative decoding algorithm which could be used after usual decoding algorithms to improve the decoding performance was brought forward，and it can correct some errors which usual decoding algorithms could detect but could not correct. The simulation result showed that the improved iterative decoding algorithm could lower the BER, especially when the SNR was low.
