Large-scale columnar jointed basalt (CJB) developments have had serious adverse effects on the stability of the project in progress to create the diversion tunnels at the Baihetan hydropower station in China. An in situ microseismic (MS) monitoring experiment was carried out to help understand the fracturing process in the CJB rock mass as a result of tunnel excavation. MS monitoring performance (accuracy of location estimation and system sensitivity) is analyzed first. Then, the spatiotemporal evolution of the MS activity associated with the Baihetan CJB rock mass fractures along the tunnel axis and at the tunnel sidewalls during the entire excavation unloading process is obtained. Finally, the forms and opportunities to deploy appropriate support are suggested according to the characteristics of the CJB microseismicity. Meanwhile, the MS monitoring methods required for CJB or jointed hard rock engineering (e.g. sensor types, sensor spacing, sensor array forms, system protection, and selection of analysis regions) resulting from this exploration experiment are summarized. •An MS test is presented for understanding excavation-induced fracturing process in the CJB.•The MS monitoring method required for CJB or jointed hard rock engineering is summarized.•Spatiotemporal evolution of MS activity during excavation of Baihetan CJB tunnel is obtained.•Timely support is able to effectively smother the aging relaxation of CJB.•Forms and opportunities to deploy appropriate support are suggested.