Structure of floor is one of the main path in which wheel-rail noise and underbody electrical equipment noise transmit to interior of the vehicle. Interior floor is one of the important parts of the whole floor structure, the reasonable design of the sound insulation perfomance is of great significance. Referring to the principle of sound insulation test of insulation material using reverberation room and anechoic room, a simulation analytical model in LMS Virtual.lab of sound insulation index of interior floor with structure of phenolic surface panel-phenolic foam core-phenolic surface panel was built. The difference of sound insulation index between simulation and test was 0.2 dB. It showed that the simulation model had a high precision predicting sound insulation index of interior floor. Using this simulation model, the results of orthogonal experimental design showed that the density of core material was the most important factor affecting sound insulation of the interior floor. The density of core material changed from 230 kg/m3 to 295 kg/m3, and the sound insulation index could meet the requirement of sound insulation index and weight constraint.