In view of high precision polishing requirements for large length-to-diameter ratio stainless steel capillary the applicable conditions of abrasive flow machining process were investigated. Abrasive flow machining (AFM) technology is a relatively new finishing machining method, making semi-solid extrusion through workpiece for machining surface to achieve the purpose of deburring, polishing, rounding and removing recast layer, which has been applied to the surface, specially shaped hole cavity, micro-hole, cross hole, bend, long tube, ceramic parts such as polishing processing. The key parameter of abrasive flow machining technology is fluid viscosity of abrasive, a suitable viscosity can ensure the smooth flow of abrasive through capillary, as well as a satisfactory polishing effect. In order to obtain feasible viscosity parameters, ANSYS Workbench Fluent was used to analyze the flow state of abrasive. The simulation results show that the fluid abrasive with viscosity of 104, 105, 106 mm2/s can not flow through capillary smoothly; that with a viscosity lower than 104 mm2/s can flow through capillary smoothly, but the inlet pressure is too high, far more than the equipment safety bearing limit; 5.585 Pa·s abrasive can smoothly flow through capillary at a relatively uniform velocity, and inlet pressure is within the scope of safe equipment load. According to the results, low viscosity fluid abrasive and special fixture were developed, and polishing experiments were conducted. A satisfactory processing effect on the inner wall of capillary was obtained. Abrasive flow machining technology is an effective method for solving stainless steel capillary inner surface polishing problems.