Time-domain reflectometry (TDR) can be utilized to assess the moisture content of building materials. However, hard building materials always have poor contact with TDR probes, which can impair the accuracy of TDR test results. In addition, in-situ factors (e.g., insertion location, insertion depth, and ambient temperature) may negatively affect the TDR test results. In order to improve the accuracy of TDR test results, this study examines the influence of the aforementioned in-situ factors on the TDR test results of moisture contents over five common building materials (i.e., concrete, cement mortar, insulation mortar, gypsum, and autoclaved aerated concrete). At first, the moisture content of these five materials is determined via the gravimetric method as the reference. Next, these materials with different moisture contents are tested through TDR, in the presence (or absence) of kaolin as the lubricating layer, from two insertion locations (front and side), at three insertion depths (1.6 cm, 2.5 cm, and 5 cm), and under three ambient temperatures (5 ℃, 23 ℃, and 40 ℃), respectively. Test results indicate that the presence of kaolin is beneficial to improving the accuracy of TDR test results, and the influence of insertion location and ambient temperature can be neglected. In contrast, the insertion depth has a noticeable impact on the TDR test results, which should be corrected using the TDR depth factor under a critical moisture content.