Electron tunneling escape time from a biased quantum well was calculated with the wave-packet method. The result agrees with the experiment reasonably, but considerable discrepancy was found when the usual semiselassical model was applied By comparillg the two approaches, the authors pointed out that the wave function can penetrate some distance into the barriers, which was ignored by the usual semi-classicla model. And thus a modified model was given to simplify the calculation of tunneling escape time.