Abstract: In order to optimize the properties of ceramic shell for superalloy investment casting, the 0.75wt% hybrid fibers were uniformly mixed in to the corundum sands via solvent method. Then, the short nylon fiber (Nsf) and short alumina fiber (Asf) modified silica sol shell were prepared with different fiber mass ratios. The microstructure of ceramic fracture was observed based on SEM, and the distribution patterns and crack growth characteristics of fibers were analyzed, which revealed the sintering behavior of fibers and matrix and further indicated the reinforcement mechanism. The results present that the properties of ceramic shell are significantly improved by a higher volume proportion of fibers in shell due to the stuccoing layer fiber addition. The flexible Nsf is wound on the surface of corundum sands, which dissipates the load energy by friction during the fiber pull-out process. Meanwhile, the Nsf is burned out and then form the in-situ holes in the matrix, which is beneficial to enhancing the permeability of shell. And the Nsf∶Asf=4∶1 sample shows the highest green strength, permeability and open porosity at 5.08 MPa, 4.4, 20.82%, systematically. In addition, the microcracks are easy to formed in the sintered ceramic shell because of the dehydration and drying However, the crack will be bifurcated, proliferated and deflected when it extends to the surface of Asf. The phenomena indicates that the interlock Asf network can effectively decrease the formation of continuous cracks, and further inhibits the ceramic particles striping and the intergranular fracture tendency for shell. Therefore, the Asf can effectively compensate the strength weaken caused by burned out Nsf. At last, the highest sintering strength of 10.51 MPa is obtained in the Nsf∶Asf=4∶1 sample, simultaneously, the high temperature self-loaded deformation rate of ceramic shell is only 0.82%.