Fiber Loop Mirror(FLM) sensing system has been widely studied in recent years, and High Birefringence Photonic Crystal Fiber (HiBi-PCF) has high birefringence, which is always the best choice for FLM sensing system(HiBi-PCF-LM). Currently, PCF is generally embedded in the fiber loop in most HiBi-PCF-LM sensing systems, and the sensor size is large. This sensing system can not be used for distributed remote measurement, and it is limited in the practical application. Therefore, HiBi-PCF-LM sensing system with an independent probe is proposed, and it provides the direction for the practical application of the FLM sensing system. However, there is no specific theoretical analysis of this sensing system in the existing research results. In order to further clarify its sensing mechanism and improve the application value of the sensing system, the interference spectrum equation are specifically deduced and the relationship between the period of interference spectrum and the length size of HiBi-PCF is discussed in this paper. It provides the theoretical basis for the practical and miniaturization of the FLM sensor.