The applications of graphene-based optical waveguide devices have been demonstrated to be one of the important directions of development for a new generation of photonic devices, and the research of graphene-based optical fiber and integrated photonic devices has attracted a great deal of attention at home and abroad. In this paper, a graphene planar optical waveguide is proposed which could transmit light by the evanescent field coupling with a microfiber. Finite element method is adopted to simulate the optical field intensity distribution and phase features of light propagating along graphene planar optical waveguide, and an experiment is performed to verify these features. Experimental results show that the transmission distribution and phases of the evanescent field are modulated by graphene obviously, it could effectively gather and transmit the high-order modes, exhibiting denser equal-phase faces on unit propagating length. In this work, we propose a new method in which the microfiber is adopted to investigate the transmission phase feature of graphene by evanescent wave coupling, which could be used as references for the design and application of graphene-based optical devices, such as modulator, filter, laser and sensor.