A lightweight design method of vehicle dash insulators is proposed and investigated in this paper. The lightweight dash insulator, which is composed of double layers of cotton felt with different density and a layer of polyethylene (PE) film and has 55% decrease in weight, is developed and applied in a passenger car, instead of the traditional “heavy layer-soft layer” dash insulator. To evaluate the NVH performance of the lightweight dash insulator, the noise reduction (NR) level index is calculated by using SEA simulation and the sound pressure level and sound qualities in the vehicle are tested under the driving conditions for wide open throttle acceleration in third gear and 60km/h cruising in fourth gear. The simulation and test results show that the vehicle with the lightweight dash insulator has better NVH performance. Compared with the traditional dash insulator, the lightweight dash insulator has higher NRs and is insensitive to the leakage effects due to the extra sound absorption of the high density soft layer, which can be enhanced by existence of the instrument panel (IP). What's more, the NVH test results of the vehicle with the lightweight dash insulator show that the interior sound quality performance is also improved obviously. The lightweight design method of vehicle dash insulators provides guidance for the sound package lightweight development and has potential applications in field of automotive industry.