Abstract: Considering that the target is an intrinsic friction medium, the cavity-expansion approximation and dynamic equations of the medium near the cavity of penetration are used to calculate the resistance of the rigid warhead. The influence of the target with a free surface on penetration is taken into account and the oblique penetration process is divided into three phases. According to the above work, the formula to compute stress on the projectile surface is deduced and a new analytical solution to oblique penetration depth of the rigid projectile into the target is obtained. The solution is identical with the BLZ formula in form and the calculation results also fit well. The solution provides the theoretical explain for the BLZ formula.