Abstract: In order to analyse the influence of composite fiber ratio on the strength and breathability of reinforced silica sol shell for investment casting, the bending strength of the embryo and after baking and breathability of composite fiber reinforced shell samples were studied. Ceramic and nylon composite fibers were used to enhance silica sol shell samples. In the slurry, the volume ratio of the nylon fiber and ceramic fiber respectively were 100:0, 82.7:17.3, 61.5:38.5, 34.7:65.3 and 0:100. The results indicate that when the proportion of nylon fibers changes from 0% to 100% in the complex fibers, the bending strength of the embryo of the shell increases gradually, the overall change of the fired bending strength is not obvious, and the breathability rate has the trend of increasing first and then decreasing. When the volume content of nylon fibers is 82.7%, the breathability rate reachs maximum, which is 5.21. According to the analysis of the fracture morphology and fiber reinforced behavior, the bending strength of the embryo of mold shell is mainly affected by the volume content of fiber and the flexural strength and permeability of the shell after baking are influenced by the volume content of ceramic fiber, the thickness of coating and the number of holes that the nylon fiber left after being burning.