Electronic and lattice dynamic properties of ternary intermetallic SrAlSi are studied with first-principles calculations based on density functional theory under high pressures. SrAlSi has a similar hexagon-honeycombed structure to MgB₂,where Sr atoms substitute Mg atoms,Al and Si atoms randomly occupy the positions of B atoms. It is found that under high pressure electronic topological transition( ETT) occur in the band of SrAlSi near Fermi surface. Lattice dynamics of SrAlSi are studied by phonon dispersion spectra at different pressures. The calculation shows that optical branches soften along A-L-H line and phonon modes harden with increase of pressure. It is indicated that structure of intermetallic compound SrAlSi is unstable under high pressures.