Abstract: The stratum structure combination of loess and red clay is common in Heifangtai area. Loess and red clay have penetration differences. Under continuous irrigation conditions,the red clay layer will have a water stagnation effect,which can cause layer softening and eventually induce landslides. In this paper,indoor geotechnical tests,NMR and particle fractionation tests were carried out respectively to study the unsaturated infiltration situation and pore change law at the interface between loess and red clay. The analysis of the test results show the following findings. (1)During the infiltration process,the peak value of the water-filled pore volume distribution map keeps moving to the right. The radius of the water-filled pore increases and the distribution range gradually becomes wider. Compared with pure loess,the maximum peak value in the water-filled pore volume distribution curve of the loess-red clay composite sample migrates more slowly to the right and has a narrower distribution range. The red clay layer is rich in water after the end of infiltration. (2)Under the same flow conditions,the higher the initial water content of the loess-red clay composite sample,the slower the peak value in the water-filled pore volume distribution curve shifts to the right,the smaller the change of the corresponding water-filled pore radius,and the water entering the medium and large. The fewer the pores,the less obvious the water stagnation effect at the interface. Under the condition of the same initial water content,the larger the flow rate,the faster the peak value in the water-filled pore volume distribution curve shifts to the right,and the corresponding water-filled pore radius is larger. The more water enters the medium and large pores,and the more obvious the phenomenon of water stagnation at the interface. (3)Particle migration occurs during the humidification process of the loess and red clay composite samples,which leads to changes in the contact mode between particles and the pore structure. The final sample contains more fine particles at the bottom and more coarse particles at the interface.