Abstract: Generally, it is difficult for high-quality source rocks to develop in marginal sags due to the influence of unfavorable factors such as small subsidence and large input of coarse clastic materials. However, the sag with moderate shielding conditions has the potential for high-quality source rocks to develop through the development of 'grit chamber' to intercept coarse debris at the edge, and then unloading fine-grained argillaceous sediments in the main depression area across the low uplift. In order to explore the sedimentary distribution of sandstone and mudstone in marginal subsags with moderate shielding conditions and promote the theoretical formation mechanism of hydrocarbon-rich subsags, this study used flume experiments to simulate the deposition and filling process according to the water supply mode of medium flow–flood–medium flow–low flow. The results show that: (1) The grit chamber and low bulge have obvious shielding effect on coarse clastic sediments, which can slow down the advancing speed of sand bodies and strengthen the differentiation of sand and mud. (2) The thickness of muddy sediments is affected by the deposition time and the intensity of reworking. The longer the deposition time, the greater the thickness of muddy sediments; the stronger the scouring effect of water flow in the later stage, the thinner the residual thickness of muddy sediments; sandy sediments can change the thickness of pre-consolidated muddy sediments in the main depression area. In the main depression area, the thickness of mud sediment is thinnest under sandy deposits and thickest at the front. (3) Moderate shielding is conducive to the development of source rocks in marginal sags. The marginal sag is sensitive to the supply of coarse clastic materials and is easy to overcompensate, while the moderate shielding is conducive to the preservation of hydrocarbon-forming materials by diverting the input of coarse clastic materials, ensuring that the main sag area receives the accumulation of argillaceous sediments for a long time, and the relatively still water body makes it easy for source rocks to develop. The experimental results provide a theoretical basis for the hydrocarbon-rich mechanism of the marginal sag, and point out that the main subsag of the marginal sag with moderate shielding conditions is a favorable area for source rock development.