The Puguang large-scale gas field in Sichuan basin has rewritten multinomial records of China marine carbonate oil and gas fields, which are the gas field of the largest scale reserves, the deepest reservoirs and the highest richness, also the one of large-scale cracked gas from oil, the driest gas and hightest H_2S-bearing. At present, the Puguang gas field developed best the secondary porosity in China. By passing further research, we found the large-scale gas field need special geological and geochemical conditions: abundant hydrocarbon source, gypsolith within reservoirs and suffering bigger burial depth (higher temperature). These conditions are necessary to TSR (thermochemical sulfate reduction) occurrence. Because of the TSR occurrence, the reaction occur acidic fluid with abundant H_2S and CO_2, meanwhile the TSR and its acidic fluids products (H_2S, etc) have strong corrosion, which alternated acutely the deep carbonate reservoir and improve the secondary poroes formation and and high quality reservoirs, So, the TSR occurrence is one of the key factors of Puguang gas field formation.