<p>The check dam is a crucial project constructed in the Loess Plateau of China to mitigate soil erosion and reduce sedimentation in the Yellow River. It features a relatively low flood control design standard. In recent years, extreme weather events have become increasingly frequent, with heavy rainfall growing more sudden and intense. This has led to severe damage to dams in the Loess Plateau, posing significant challenges to the safety and governance of the Yellow River Basin. To address this issue, this study utilizes loess from the northern Shaanxi Loess Plateau as the dam-building material and employs Anionic Polyacrylamide (APAM) to reinforce loess check dams. Through multiple tests including electron microscopy, soil triaxial tests, nuclear magnetic resonance (NMR) tests, and flume model tests, the mechanical properties and structural characteristics of modified loess are analyzed from multiple dimensions, revealing the physical mechanism of overtopping failure in APAM-modified loess check dams. The results indicate that: incorporating APAM can enhance the strength of compacted loess, and the strength increases with the rise in APAM content; APAM can cement fine loess particles into larger aggregates, reducing the number and area of pores in the internal structure of compacted loess and significantly improving the soil’s erosion resistance; the modified compacted loess mixed with APAM can greatly alleviate the overtopping failure degree of check dams, prolong the dam failure time, and effectively reduce the breach flow rate and expansion rate. The research findings can provide theoretical support for the study of overtopping failure mechanisms and disaster prediction of compacted check dams.</p>