The photon-number squeezed light generation using a nonlinear polarization interferometer (NOPI) is analyzed numerically. The NOPI is constructed by the cascade connection of 90 degree twisted couple of polarization-maintaining fibers. Since it is possible to generate photon-number squeezed light by only passing an optical pulse through an optical fiber without spatial alignment, the squeezed light can be generated easily and stably in this scheme. The evolution of the quantum noise for the optical pulse spectra is calculated using the quantized nonlinear Schrödinger equation and linearization approximation. The magnitude of maximum squeezing in the numerical analysis is estimated to be about —13 dB. Since the arbitrary dividing ratio and two outputs can be obtained by merely turning the half-wave plates on the input and output side, photonnumber squeezed light can be generated for the broad input and output power ranges. It is clarified that the generation mechanism is contrastive for the turning direction of half-wave plate on the output.