Abstract: In order to analyze the influence of fiber length on the bending strength and breathability of the ceramic fiber reinforced silica sol shell, the ceramic fibers with six different lengths from 1 mm to 6 mm were added to the silica sol slurry to reinforce shell. The results show that the length of the ceramic fiber has a significant effect on the strength and breathability of silica sol shell. With the increase of the ceramic fiber length, the bending strength of the shell increases first and then decreases, and the breathability of the shell increases first, then decreases and increases again. When the fiber length is 4 mm, the green strength and fired strength of the shell reach the maximum about 2.97 MPa and 6.84 MPa, respectively, and the breathability of the shell also reaches the maximum about 2.90. When the fiber length is more than 4 mm, the dispersion of fiber in shell gets worse, and the influence of fiber bridging is not obviously effective. The porosity of the shell reduces, which leads to the decrease of the strength and breathability of the shell.