The flying and distributing characteristic of the fragments was analyzed. The engineering calculation method for the lethal probability of the blast-fragmentation warhead exploding in the air was introduced. By this method, the contours of the lethal probability and fragments hit density were shaped, the curves of the damage area varied with the exploding height were described at the different lethal probability. The results show that the damage area has a peak value when v_c=700 m·s^-1, θ_c=70°, and the lethal probability is equal to 0.5 or 0.8. The peak value is corresponding to the optimal exploding height of 8 m. The curves of fragments hit density on the different direction of the warhead are coincident with the experimental value.