Groundwater and mud inrush disaster from completely weathered granite presents a huge difficulty for tunnel construction, which requires the grouting measurement with favorable performance. To propose the optimal material parameters for grouting, numerous tests, including strength, permeability, and anti-washout, were conducted to evaluate the effects of grouting filling ratio (GFR), curing age and water velocity on the grouting effect. The test results show that: (1) The hydraulic property of completely weathered granite can be significantly improved by increasing the grouting volume and curing age. In particular, when GFR ≥ 48%, the cohesion and internal friction angle increased to about 200 kPa and 30°, which were more than three and ten times of that pre-grouting. (2)With the increase of GFR, the permeability exhibited three stages: Slowly decreasing stage, sharply decreasing stage and stable stage. When increased from 32% to 48%, the permeability coefficient sharply decreased two orders of magnitude, namely from 4.05 × 10 cm/s to 1 × 10 cm/s. (3) The particle erosion rate decreased sharply to below 10% in the low water velocity (v ≤ 0.2 m/s) when GFR ≥ 48%, but still exceeded 50% when v ≥ 0.4m/s. The results indicated that the grouting volume of GFR = 48% was a suitable grouting parameter to reinforce the completely weathered granite, particularly in the low water velocity condition. The field investigation of hydraulic-mechanical behaviors in the Junchang tunnel indicated that the grouting effect can be improved markedly.