The structure of loess is loose, and the shear strength of loess drops sharply after contact with water. Therefore, loess mudflows have become a common geological disaster on the Chinese Loess Plateau. In order to study the initiation mode and mechanism of loess mudflows, in this study, seven sets of flume experiments were designed by controlling the slope angle and rainfall intensity. The results show that (1) when the slope angle is between 10° and 20°, there are two initiation mechanisms of loess mudflows: mudflow (large scale) and retrogressive toe sliding, and mudflow (small scale) and retrogressive toe sliding. (2) The main method by which water infiltrates into the soil accumulation is mainly vertical infiltration, which is not affected by the slope angle and the seepage direction of the accumulation soil. (3) The liquefaction of loess is the root cause of loess mudflows. Water infiltrates into the area with an uneven density and a large amount of water accumulates in this area. Thus, the water content of the loess increases and the pore water pressure increases quickly and cannot dissipate in time, so the loess liquefies and the liquefaction area continue to spread and become larger. Thus, loess mudflows (large scale) occur. The increase in pore water pressure was captured in the seven sets of experiments. However, the order of the rising positions in the accumulation were different. This requires us to carry out tracking of the particle displacement inside the soil and the spatial changes in the internal structure of the soil in future research.