Simulated coal gas (SCG) was used as pyrolysis atmosphere to improve tar yield and quality of coal pyrolysis, and the effect of SCG compositions including H-2, CH4, CO, CO2 on pyrolysis products was also investigated. The results show that tar yield of coal pyrolysis is obviously enhanced under SCG atmosphere compared with that under N-2. The components in SCG apparently influence the distribution of pyrolysis products and play a different role in coal pyrolysis. The highest tar yield (16.3 wt%) is obtained under SCG atmosphere at 650 degrees C. H-2 reduces the char yield by suppressing secondary reaction; coexistence of H-2 and CO2 in pyrolysis atmosphere increases water yield significantly via reverse water gas shift reaction, but the addition of CO inhibits this reaction. The analysis of tar by simulated distillation and GC/MS showed that the addition of CH4 increases the contents of benzenes and phenols; the introduction of H-2 mainly increases naphthalenes; while the introduction of CO inhibits the formation of phenols. Lower aromaticity of tar under SCG than that under N-2 indicates that small molecular radicals (i.e. center dot H, center dot CHx radicals) generated from SCG participate in the stabilization of free radicals from coal pyrolysis, resulting in high tar yield and quality.