The flexible extrusion forming process (FEFP) is a sand mold patternless manufacturing technology that enables digital near-net shaping of complex sand molds. But, it is difficult to achieve the gradient sand molds with high surface strength and strong interior permeability by FEFP. To solve this problem, an extra-squeeze forming method based on FEFP for gradient sand mold was developed. To further reveal the extra-squeeze forming mechanism, based on the Johnson-Kendall-Roberts (JKR) theory and “gluing” notions, the single and double-sided squeeze models of gradient sand molds were established using the EDEM software. The squeezing processes of sand molds with different cavity depths of 60, 100, 140, 180, and 220 mm were systemically studied under single and double-sided squeeze conditions. The variation in the void fraction of sand mold as also investigated at a variety of extra-squeeze distances of 2, 3, 4, 5, and 6 mm, respectively. Simulation and test results show that a deeper cavity depth weakens the extrusion force transmission, which leads to a decrease in strength. The sand mold permeability and void fraction are identified to be positively correlated, while the tensile strength and void fraction appear to be negatively correlated. The void fraction of sand molds decreases with a longer extra-squeeze distance. A 6 mm extra-squeeze distance for the sand mold with 220 mm cavity depth results in a 26.8% increase in tensile strength with only a 5.7% reduction in the permeability. Hence, the extrasqueeze forming method can improve the quality of the sand mold by producing a gradient sand mold with high surface strength and strong interior permeability.