Chip shape is an important factor affecting the processing quality of the deep hole gun drilling, because the hole is too small and deep. The relationship between chip forming mechanism and process parameters has long been a difficult problem in deep hole gun drilling research. Taking 15-5PH stainless steel as the research object, the research explored the process and influencing factors of spiral chip forming by analyzing the chip flow process. The fracture mechanism of 15-5PH stainless steel gun drilling chips was studied with scanning electron microscope. The chip deformation rule under different process parameters was studied through experiments with varied cutting speed, feed, and oil pressure. The relationship between chip deformation and oil pressure was clarified by analysing fluid pressure and turbulent momentum on the cutting edge of gun drilling under different oil pressure with ANSYS-FLUENT. The study shows that the fracture mechanism of chips processed by 15-5PH stainless steel gun drill belongs to “quasi-cleavage fracture” and the chip morphology is closely related to the technical parameters of the gun drill. The research results can provide guidance and references for the selection of processing parameters and coolant oil pressure of deep hole gun drilling of 15-5PH stainless steel.