For joint source channel coding (JSCC) scheme based on double protograph low-density parity-check (DP-LDPC) codes, due to joint iterative decoding between the source and channel decoders, it is found that the optimal channel code in separate system is not optimal in joint system. In this letter, a novel joint protograph extrinsic information transfer analysis is proposed for the DP-LDPC system over AWGN and redesigning scheme for channel code is put forward to enhance the performance of DP-LPDC system. Both the EXIT analysis and the simulated results show the performance improvement of proposed systems. This verifies the viewpoint that the channel code should be redesigned for JSCC scheme even if the optimal channel codes are selected over AWGN in separate systems.