Mass loss should be considered while calculating the penetration depth of concrete by eroding long-rod projectiles of high velocity. The penetration process is divided into two phases: eroding phase and rigid phase. During eroding phase, a model to predict the penetration depth is established on the assumption that there is a chipping region in the bottom of crater. During rigid phase, Forrestal formula is adopted to calculate the penetration depth. Using this model, the depth of concrete penetration by a tungsten alloy long-rod projectile is calculated. When the critical eroding velocity is between 950 m/s and 1 000 m/s, the result is in good agreement with the experimental data.