Relay is considered as one of the candidate key technologies of LTE-advanced for it brings broader cell coverage and higher system capacity. Distributed Space-Time Coding (DSTC ) not only exploits the spatial diversity from the relay structure, but also brings some coding gains. According to the performances of existing DSTC schemes, the Demodulate-and-Forward (DMF ) scheme is the best candidate for DSTC in LTE-Advanced system. Moreover, a threshold-based selective relaying scheme for DSTC is proposed to solve the error propagation of DMF. This scheme can achieve excellent performance gains at the cost of little system complexity.