The oncolitic limestone of Chuanshan Formation is an important building material widely distributed in the middle and lower reaches of the Yangtze River and southeastern coastal provinces. Based on the theory of fracture mechanics,this paper discusses the process of micro-crack propagation and macro-fracture of nuclear limestone under tensile stress by polarized light microscope and scanning electron microscope. Studies have shown that there are microcracks at the interface of oncolite particles,secondary pores,intergranular pores,etc.,and micro-crack propagation needs to consume energy. When the crack expands near the oncoids,there are two different paths to bypass the oncoids or pass through the oncoids,which mainly depends on the structural factors such as the type of oncoids,structural density,and particle size.