<p>Assessing the Geological Strength Index (GSI) of subsurface rock masses often presents challenges to inexperienced engineers, primarily due to the limited exposure of discontinuities in the rock masses. This study combines data from 12 drilled cores from Maerdang and Shanyang hydropower stations, including their Lugeon test results. The statistical results show the GSI of the rock mass rises with depth and falls with an increase in Lugeon values, both these relationships are weak correlations. The correlation between the GSI and Lugeon values is significant only when the Lugeon values surpass a specific threshold. There exists difference in the Lugeon value threshold and the correlation between GSI and Lugeon values in monzonite and metamorphic sandstone. For monzonite, a power-law relationship is observed when the Lugeon value exceeds 2, a linear relationship arises for metamorphic sandstone when the Lugeon value surpasses 0.5. On the basis of above, the research develops GSI prediction equations for monzonite and metamorphic sandstone using multiple regression methods. Given the limited data from only two projects, more research is needed to validate the wider application of the GSI prediction equation.</p>