We investigated the incoherent optical modulation of graphene based on an in-line fiber Mach-Zehnder interferometer fabricated by offset splicing of a D-shaped fiber and single-mode fibers. The D-shaped flat surface was coated with a single-layer graphene whose refractive index can be modulated by an external laser. With a backward pump-probe setup, we found that the incoherent optical modulation occurred independently of the wavelength difference between the pump and probe lasers, supporting the theoretical analysis that the incoherent optical modulation is mainly contributed by the carrier-phonon process of graphene. Further, both the decrease of the pump wavelength and the increase of the pump power would deepen the modulation in an exponential trend. The experimental results are beneficial to understanding the incoherent optical modulation process in graphene. The proposed interferometer covered by graphene can also be used as a compact all-optical intensity modulator.