Compound modification of Al-25% Si alloy with P-Cr-Ti is utilized to study the change of the solidification microstructure and the effect mechanism of Cr and Ti elements. The results indicate that primary Si in the solidification microstructure decreases by 12.24%~51.65% in the Al-25% Si alloy modified by the compound modifier P-Cr-Ti compared with simple phosphorus modification, and the distribution of the primary Si is more uniform. The Cr and Ti elements in the Al-25%Si alloy mainly appear as compounds including TiAl₃, Ti₇Al₅Si₁₂, Al₇Cr and Al₁₃Cr₄Si₄, while a minor amount of P exists in the compounds containing Ti. Compounds containing Ti appear as long strips. Compounds containing Cr, which distribute among the primary Si, appear as granular or short rods. The number of the compounds containing Cr and Ti is increased with the increase of the cooling rate. The TiAl₃ phase initiated by Al-6.5%Ti alloy wanders in the melt during the modifier process. The {110} crystal of TiAl₃ phase becomes the nucleation basal of the Si phase, causing the primary Si to precipitate continuously and to refine the primary Si with a limited refining ability. Compounds containing Cr formed among the primary Si prevent the growth and gather of the primary Si during the solidification, which leads to the distribution of the primary Si more even.