Magnesium alloy is light, and has large specific stiffness, great damping and excellent shock absorption. It had been made into automotive dash successfully. In order to avoid severe requirements of theoretical model and experiment condition limits, sound insulation performance of dash could be obtained by structure-sound interaction method. This approach was validated by comparing the numerical solutions with experimental results. Sound transmission loss of automotive dash in diffuse field was computed. The results indicated that in low frequency band, comparing with steel based dash, sound insulation performance of magnesium alloy based dash is much better. Replacing steel by magnesium alloy, original structure of dash needs to be improved, and its eigenfrequency should be beyond the peaks of noise source, avoiding deterioration of acoustic characteristic in certain frequency bands. Applying magnesium alloy to vehicle could lose weight as well as improve cab’s acoustic environment.