JX and JXR series phosphate self-forming core materials used to fill narrow cavity in investment casting shell mold have been investigated including slurry fluidity and suspending stability, baked strength and high temperature stability as well as residual strength of the filler. In addition, JX-2 and JXR-2 materials have been compared with optimized JXR-3 for filling. The results show that the properties of JXR-3 is superior to that of JX-2 and JXR-2 in condition of meeting high fluidity, exhibiting desirable working time and suspending stability, higher baked strength and higher temperature stability. The sol silicate slurry mixing is the best in three kinds of slurry mixing including water, sol silicate and phosphate acid solution. The experimental results verified that the JXR-3 slurry is suitable for filling narrow cavity in investment casting shell mold, effectively avoiding the casting swelling and metal-leakage defects in cavity.