Rock macro mechanic character is decided by rock composite and meso configuration. LuHui granite minerals composite, micro configuration and the rock thermal cracking in differ temperature are observed by micro-photometer in meso. LuHui granite is composited by kinds of minerals, and its minerals’ inhomogeneity is visible, the change of cementation among rock crystal grains, dislocation and micro cracking in crystal is produced in crystal grains under temperature. By means of experiment, the development of granite micro cracks and interior configuration, and change laws of micro cracks were observed. The quantitative analysis of the micro mechanism and laws of granite thermal crack was made, granite cracks number increases acutely in 240-260 degree C after a narrow range fluctuation in 80 degree C. Finally, the thermal cracks threshold of granite was determined primarily.