Based on problem of the thick sandstone roof could not caving along with mining workface advancing of 3 # coal seam in a mine, 7603 work face was chosen as engineering background together with adopting comprehensive application theoretical analysis, numerical simulation, industrial test methods, to name but a few. Technologies of initial caving step reduction and weakening basic roof in thick sandstone of fully mechanized caving face were studied and roof pre-splitting technique scheme was taken to conduct industrial test in 7603 working face. The field application showed that beginning of step distance apart of workface and first roof weighting pace in old roof were reduced directly and effectively on the basis of carbon dioxide gas fracturing technology to weaken the working face roof, which ensured safety production of working face during preliminary mining.