Phase behavior of the reflected terahertz radiation (THz) under surface plasmon resonance (SPR) supported by doped graphene has been comprehensively investigated via theoretical analysis with simulation verifications. For a TM-polarized wave, the dependence of the phase on the angle of incidence has a region with an abrupt jump-like change. We found in particular that the resonance phase dependence would change from step-like contour to Fano lineshape when the system passed through the optimum SPR conditions (i.e., R = 0) in terahertz regime. Monitoring the transformation could provide ultrahigh-sensitive label-free detection of biomolecules. Importantly, the characteristic of phase jumps as a readout response to achieve refractive index sensing that outperforms traditional terahertz-amplitude-based attenuated total reflection (ATR) spectroscopy is valuable. The results demonstrated a high figure of merit (FOM) of up to 171, based on the terahertz phase information. Moreover, the sensing range could be tuned by changing the surface conductivity of graphene via high doping levels or with few-layer graphene. These terahertz phase response characteristics of graphene plasmon are promising for tunable ultra-sensitivity (bio)chemical sensing applications.