Using Ar as buffer gas, the collisional influence on two different population ways of high lying state in Na 2 ladder level system is studed experimently. For two photon transition, because of dephasing collision, Doppler free Lorentz peak will vanish via the increasing of buffer gas pressure. Injecting little buffer gas to the cell will enhance the Doppler width signal because of collisional excitation, but too much buffer gas will decrease the signal because of population eliminating effect. For collisional high lying state population, injection of buffer gas will be the key function to high lying state population. The explanation in the term of rate function has been given.