The "filter cake sandwich" phenomenon is a typical problem encountered in the dewatering process of coal slurry using frame filter presses in coal preparation plants. By employing a self-designed frame filter press testing machine and numerical simulation techniques, this study investigates the optimal feeding conditions under current equipment settings, including the formation process of the filter cake, filtrate concentration, dewatering rate, particle size distribution of the filter cake, and moisture distribution; the migration patterns of the solid and liquid phases within the filter chamber; and the influence of filter chamber thickness on the internal moisture distribution of the filter cake. The study also explores the growth behavior of the filter cake and the underlying causes of the "sandwich" phenomenon.Experimental results indicate the existence of a critical feeding pressure and an optimal feeding concentration. Under these optimal conditions, the filtration time is 8 minutes, with an average moisture content of the filter cake at 23.44%. The filter cake formation process is divided into three stages: base accumulation, structural formation, and internal filling. During the frame filtration process, the "sandwich" phenomenon in the filter cake primarily occurs in the upper-central region of the filter cake, which is the last region to form and the most challenging for filtrate discharge. The particle size distribution of the filter cake is uneven, with coarse particles mainly concentrated at the bottom and central layers, while fine particles are predominantly located in the upper-middle and surface layers. This uneven distribution further impedes the discharge of filtrate from the central layer, which is a critical factor contributing to the "sandwich" phenomenon.Numerical simulation results demonstrate that under optimal conditions, both the solid and liquid phases migrate smoothly, allowing for the complete discharge of filtrate from the central region, thereby effectively preventing the "sandwich" phenomenon. However, when the filter chamber thickness increases to 38 mm, the "sandwich" phenomenon begins to manifest and progressively shifts from the feed layer toward the center. As the thickness of the filter chamber increases, the liquid phase volume fraction in the central region increases, leading to significant moisture retention.