In this work, we demonstrate and fabricate a GaN-based quasi-vertical Schottky barrier diode (SBD) with the sidewall field plate structures (SFP) on sapphire substrate. The fabricated SFP-SBD with a 3 μm drift layer exhibits excellent reverse blocking characteristics, e.g., the low leakage current is maintained at ~10⁻⁹ A/cm² until hard breakdown occurs, and the breakdown voltage (BV) can reach ~420 V. The enhanced reverse blocking characteristics can be well attributed to the decreased local strong electric field at Schottky contact interface and electrode edge with the help of SFP structures. Meanwhile, the simulation results present that the thick field plate insulator, e.g., 1 μm SiO₂ in this work, can share a large part electric field from the Schottky contact interface in the condition of high reverse bias, which well reduces the leakage current. Moreover, the SFP structure does not make a marked degradation for the forward conduction characteristics, e.g., a turn-on voltage (V_on) of ~0.6 V and a specific on-resistance (R_on,sp) of ~10 mΩ·cm² can also be obtained for the fabricated SFP-SBD in this work.