In the presence of narrowband interference (NBI), the ranging receivers should not only remain steady but also satisfy the requirement of ranging precision. By taking into account both the system stability and the precision requirement, this paper defined the anti-jamming margin of a ranging system (RS-AJM) to provide a comprehensive evaluation of anti-jamming capability. A margin reference factor was introduced to describe the relationship between the two requirements, which was used to divide the margin calculation into three cases. When the system is in the high stability region, the system stability is more important than the ranging precision. When the system is in the high precision region, the anti-jamming margin mainly depends on the precision requirement. When the system is in the middle region, the two requirements determine the anti-jamming capability together. The middle region can be further divided into another two zones according to the center frequency of NBI. The margin reference factor and the performance scenario division present a clear and comprehensive description of the anti-jamming capability, which is also verified using numerical results.