The present invention applies to the digital signal processing field, provides a mapping and interlacing method and system for digital television terrestrially broadcast signals. The method includes: acquiring interlacing mode information and modulating mode information corresponding to the input LDPC coding bit data; interlacing the LDPC coding bit data according to corresponding interlacing mode based on the interlacing mode information; based on the modulating mode information, when the modulation mode is one of 64QAM, 32QAM, 16QAM and 4QAM, directly performing constellation mapping of corresponding modulating mode to bit data after interlacing, and when the modulation mode is 4QAM-NR, firstly performing NR quasi-orthogonal pre-mapping and then performing 4QAM constellation mapping to the bit data after interlacing. The present invention, by firstly interlacing and then mapping the bit data stream together, decreases the bits of basic symbols during interlacing, can decrease the size of memory of the interlacing device, and does not require to increase additional module to realize according to the 4QAM-NR mode, thereby reducing the production cost.