The measurement accuracy of ultralong fiber interferometers is mainly limited by the 1/f noise in the ultralow frequency band from mHz to kHz. The sources and distribution principle of 1/f noise need to be thoroughly studied to suppress measurement noise and improve measurement resolution. For this reason, a balanced polarization-maintaining fiber Mach-Zehnder (M-Z) interferometer was built for differential detection, in which the arm length is over 100 m. By testing optical fibers with different lengths as well as different types of narrow line-width laser utilizing this interferometer, we obtained the distribution characteristics of ultralow-frequency phase noises respectively induced by laser frequency drift and fiber thermal noise on the spectra. According to the experimental results above, the distribution principle of 1/f noise in the ultralong fiber interferometer was also explained. The results show that the amplitude of phase noise increases rapidly from the order of 1 μrad/Hz ^1/2 @1 kHz to the order of 10 mrad/Hz ^1/2 @1 mHz within a 260 m arm-length M-Z interferometer, which is due to the laser frequency drift and fiber thermal noise. It can be concluded that for the frequency band from mHz to kHz, the phase noise of the ultralong fiber interferometer satisfies 1/f^1+β spectral distribution.