We design a terahertz monolayer graphene-based modulator using the property of voltage-tunable optical conductivity of graphene. In order to enhance the graphene-terahertz interaction strength, a metal-graphene hybrid structure is used to construct the modulator. The full-wave electromagnetic simulations show that the modulation depth is larger than 90% at 3.5 THz in the reflection operation mode. The prototype device is fabricated with planar semiconductor fabrication processes. The reflection spectra of the device are measured. The experimental data are in good agreement with the simulation results. The method is helpful for realizing high performance terahertz modulators.