The underground cavern of Baihetan Hydropower Station requires a significant amount of concrete, with a combination of basalt artificial sand and basalt coarse aggregate being adopted for the concrete aggregates. To ensure the durability of the concrete in the underground cavern, the alkali reactivity of the basalt aggregate was monitored quarterly from November 2017 to August 2021 during the construction of the Baihetan Hydropower Station. Various test methods, including the petrographic method, mortar bar rapid method, and concrete prism method, were employed to evaluate the potential alkali reactivity. The experimental findings revealed the presence of other minerals such as amygdaloidal basalt and breccia lava in the basalt aggregates. Additionally, a small amount of active minerals like microcrystalline and cryptocrystalline quartz was identified. SEM and EDS results indicated that these active minerals would only induce harmful alkali aggregate reactions under high temperature and high alkali conditions. Utilizing low-heat Portland cement with an alkali content of less than 0.60% and incorporating a minimum of 20% fly ash in the concrete mixture effectively mitigated the risk of harmful alkali aggregate reactions associated with basalt aggregate.