In this paper, a creative and novel composite waveguide based monolayer graphene-clad microfiber (MGCM) device was designed to achieve ultra-broadband (730～1 700 nm) microfiber waveguide all-optical modulator. We took the biconical microfiber from a standard telecom single mode fiber with flame biconical taper method with low transmission loss, which enhances the evanescent wave and material interaction on the microfiber surface. Using the super-features of graphene, such as atomically thin, linearly dispersive band structure, ultra broadband light-matter interaction, strong interband transitions and ultra-fast carrier relaxation time, monolayer graphene was wrapped as a saturable absorber on the cone of the biconical taper fiber to enhance the interaction between the evanescent wave and monolayer graphene on the surface of the composite waveguide. Both the static and dynamic all-optical modulation experiments were carried out and we achieved supercontinuum (480～1 700 nm) light modulation by chopping the conventional low-power CW laser diode (808 nm) with pump power under 50 mW and the modulation depthsare more than 5.7 dB, the modulation speed is measured to be ~4 kHz. The all-optical modulator based on microfiber composite waveguide achieves ultra-broadband all-optical modulation with lower pump power while ensuring the depth of modulation. It was compatible with the current high-speed optical fiber communication networks in a simple, effective and inexpensive way, which paes the way for to micro/nano ultrafast optical signal processing.