Gallium oxide (Ga2O3) is the promising material for the solar-blind photodetector (SBPD) because of its ultra-wide energy band gap. In this paper, the metal-semiconductor-metal (MSM) SBPDs were fabricated based on the high quality single crystal β-Ga2O3 nanobelts grown by carbothermal reduction without catalyst. The influences of annealing treatment for devices were investigated. The increase of annealing temperature (Ta) improved the SBPD's performance greatly. Ultra-low dark currents of ∼0.89 pA and ∼3.58 pA at 20 V were achieved after Ta = 500 °C and 550 °C annealing process, respectively. Meanwhile, at the optimal Ta of 550 °C, the device achieves excellent performance: high photo-to-dark current ratio of more than 106, high responsivity (867 A/W), high external quantum efficiency (4.2 × 105%), and high detectivity (1.8 × 1015 cmHz1/2W−1 (Jones)).