Abstract: Considering material dispersion and tunneling ray, the D.Gloge’s theory formula of time response of multimode step-index fiber is modified. The modified formula is used to calculate the function of time response of 50 m long, 0.2 mm core diameter step-index fiber with scintillator illumination. Results show that it is near a Gaussian distribution with FWHM 3.0 ns, the pulse risetime is more than the falling time and the falling time has a trailing.Time response of scintillator-fiber system is modeled by numerical method, the results of modeling accords with experimental results.