At present, aiming at the problem of many trapping sets are generated when applying the traditional method to construct (3,m) LDPC codes,this paper proposes an improved scheme to construct (3,m) LDPC codes,which eliminate elementary trapping sets(ETS) based on rectangle lattice.By analyzing the relationship between ETSs and slopes,we can select the proper slopes which satisfy the corresponding constrained condition so as to avoid the emergence of ETSs;Simultaneously,quasi-cyclic structure is applied to the parity-check matrix in optimized scheme,which reduces the complexity of encoding and decoding for LDPC codes.The results of simulation experiments on the AWGN channel demonstrate that the proposed constructional algorithm in this paper can reduce the error floor effectively.