Abstract: Since Variable-Length Codes（VLCs） are inherently vulnerable to channel errors，traditional error-correcting codes are unable to solve the propagation-error problem efficiently.A new symbol-constrained joint source-channel decoding algorithm for VLCs is presented.By constructing double-tree VLCs sources，the propagation-error problem is evidently alleviated.Meanwhile，by applying symbol-constrained method to MAP algorithm，not only the a priori probability of VLCs sources is efficiently utilized，but also the Symbol Error Rate（SER） is reduced enormously.Compared with traditionally bit-constrained MAP algorithm，simulation results demonstrate that for JPEG source over AWGN channels，the gain in SNR of the method is around 0.8dB when SER reaches 1%.