Studying on microstructure of PBX chips and its formation mechanism is helpful to understand the forming process of machined surface. It′s of a great significance to guarantee and improve machining surface quality. The laser particle size analyzer was employed to detect PBX chips samples. It shows that the average size of chips which prepared by turning with cutting fluid is larger than that prepared by dry turning，the size increases with feed and decreases with the elevation of machine spindle revolutions，and has no obvious relationship to turning depth. Through the observation via scanning electron microscope，we can discover that chips mainly have three types of micro-shapes like acicular，massive and squamose，its formation corresponds to PBX macro-fracture which caused by the nucleation and growth of crack and mainly embodied as transgranular fracture of explosive particles. The detection of PBX samples sub-surface through industrial CT suggests that the crack propagation path points to workpiece surface generally，it relates to relative movement state between cutting tool and workpiece，and relates to structural boundary conditions of workpiece too. The phenomenological chip formation model can help to master its formation mechanism.