The mathematical mode for reflectance and transmission of the fiber-optical intrinsic unsymmetrical Fabry-Perot interferometer (IUFPI) is presented. The mathematical model for fiber-optical cavity longitudinal change of a low fineness IUFPI and intensity of the interferometric lignt is also presented. If r′1=t1t′1r2=r, and t1t′1=1-r′21≈1, then the interference of reflections from fiber optical IUFPI can be approximately regarded as interference of two beams with equal amplitude. In the same time, the mathematical model for output of optical-electronic converter and the interference intensity of two beams with equal amplitude is discussed. All the mathematical models are the foundation of fiber-optical IUFPI applications.