An in situ crystallized FCC catalyst with high porosity and high Y-faujasite percentage was successfully synthesized via adding a lab made porosity enlarging agent (PEA) to kaolin clay. The in situ crystallized microspheres (CMS) had a pore volume of 0.462 mL g⁻¹, a specific surface area of 406 m² g⁻¹, and a Y-faujasite content of over 60%. Densely packed Y-faujasite crystals and abundant macropores can be observed on the surface of the in situ crystallized microspheres. After ion exchange with NH₄NO₃ solution and rare earth lanthanum, the crystallized microspheres had an attrition index of 2.2% h⁻¹, meeting the commercial requirements of FCC catalysts. The ratio of Brønsted sites to Lewis sites was 2.8, and the ratio of weak, medium and strong acid sites was 1.83 : 1.33 : 1. The resulting catalyst had a good hydrothermal stability. Its catalytic performance in cracking mixing oil displayed high selectivities to gasoline and light oil, and low selectivities to dry gas and coke.