A large number of loess landfills have been generated by the Pingshan land formation project carried out in Lanzhou area, which has a large safety hazard due to its low compaction and lack of necessary protection, resulting in the extensive development of geologic hazards such as loess caves, landslides, and so on. The pore microstructure of in-situ loess landfill with different water content was quantitatively studied through scanning electron microscope and Image J software, combined with fractal theory, the pore type, number, area, and change rule of the number of dimensions of the pore of the landfill loess with different water content was obtained, and the dynamic relationship between the pore structure and the wetting of loess and development of the caves were preliminarily analyzed. The results show that: with the increase of water content, the number and area of large and medium-sized hollow pores gradually decrease, while the number and area of small hollow pores decrease, but the area of small hollow pores increase, and the collapse of large and medium-sized hollow pores is the main reason for wet subsidence and deformation of loess; the pore dimension number of landfill topsoil has a linear negative correlation with water content, and a positive correlation with wet subsidence; the average pore dimension number of in-situ topsoil is 1.251, and the average pore structure is 1.251. The average pore dimension is 1.251; based on the characteristics of cave development in loess landfill, it is proposed that the protection treatment should be carried out in three aspects, such as the construction of drainage ditches, the reinforcement of caves, and the protection of slopes. The research results can provide theoretical support for the engineering construction and geologic disaster prevention and control research in Lanzhou landfill loess area.