The effect of cooling rate, undercooling degree and recalescence temperature on the morphology of the primary Si-phase and the mechanical property of Al-20%Si alloy were investigated by means of high-precision thermometer, optical microscope (OM) and scanning electron microscopy (SEM). The results showed that the average size (D) of the primary Si in Al-20%Si alloy is a power function of the cooling rate (v) as D=260.6v^-3/4, and linearly related to the recalescence temperature (T_m) as D=0.25T_m-143.12; Reducing the recalescence temperature of the primary Si growth was the key to control the grain growth, the copper mold with high thermal storage coefficient may be favourable to the sustainable reduction of the nucleation temperature and recalescence temperature of the primary Si, so that the primary Si size was small; The critical supercooling degree of 70 K was needed for the transformation of the primary Si growth from facet-like to non-facet-like ones, which is consistent with the theoretical calculation (74 K). With the increase of cooling rate and the undercooling degree, while the decrease of recalescence temperature, the solidified microstructure of Al-20%Si alloy was refined remarkably, correspondingly, the tensile strength of the Al-20%Si alloy increased from 167 MPa to 210 MPa and the elongation increased from 2.14% to 3.89 % respectively.