We present a low-complexity high-efficiency LDPC encoder, based on classic method of Richardson and Urbanke with a novel backtracking algorithm, which we propose to substitute greedy algorithm for approximate triangulation with sparse matrix of LDPC codes. For the LDPC code in CMMB, for example, the complexity of encoding is reduced effectively, and an implementation of LDPC encoder for two different code rate (1/2 and 3/4) on Altera Stratix II EP1S180F102014 can achieve encoding rate 34 Mbps and 69 Mbps.