It is crucial to include additional site amplification effects resulting from the thick sediment on ground motions in the reliable assessment for seismic hazard in sedimentary basins. Ground‐motion residual analysis with respect to ground‐motion prediction equation is performed to evaluate additional site amplifications at over 200 K‐NET stations within and around Kanto basin. We first investigate the potential effects on additional site amplifications resulted from the sediment depth and several source‐dependent factors. Results reveal that source‐to‐site distance, focal depth, and source azimuth all have nonnegligible effects on additional site amplifications, especially the focal depth. Thick sedimentary sites amplify long‐period ground motions from distant earthquakes more strongly than those from local earthquakes. Ground motions from shallow crustal earthquakes generally experience much stronger amplifications than those from those deep subduction earthquakes, much more predominant for long‐period ground motions (>1.0 s) at thick sedimentary sites. Meanwhile, we develop the empirical model after integrating contributions from sediment depth, source‐to‐site distance, and focal depth for predicting additional site amplification effects. Considering the typical case of the distant shallow crustal earthquakes, additional site amplifications at thick sedimentary sites within Kanto basin generally show an increasing trend with the oscillation period increased, whereas they are generally characterized by a decreasing trend at shallow sedimentary sites outside the basin. The mean additional site amplification is up to about 2.0 within Kanto basin, whereas 0.5–0.65 outside Kanto basin, for ground motions at oscillation periods of 2.0–5.0 s. Mean amplifications within Kanto basin are about 3.5 times larger than those outside the basin for long‐period ground motions at 2.0–5.0 s. Sites northeast to Kanto basin show the largest amplifications up to about 3.0 at periods of 0.15 and 5.0 s, which may be resulted from the basin edge effects.