To investigate the mechanical response characteristics of debris flow deposits under impact load, a combination of geotechnical tests on deposits, similar model tests under impact load, and numerical simulation was adopted to analyze the time-history curves of acceleration, velocity, displacement, and stress of cylindrical projectiles penetrating debris flow deposits, as well as the evolution characteristics of motion attitude, resistance, and cavity during the projectile penetration process. The experimental results show that when the projectile penetrates the debris flow deposit, the displacement increases rapidly. With the instability of the projectile’s motion attitude in the debris flow deposit, the deceleration gradually increases. When the axial direction of the projectile is perpendicular to its motion direction, the contact surface is the largest, the resistance reaches the maximum, and the negative acceleration also reaches the maximum. During penetration, the projectile displaces the debris flow deposit to the surrounding area to form a large cavity, and the velocity of the projectile decreases while the displacement tends to slow down. With the increase of time, the projectile flips over, and finally the tail faces forward. Additionally, the contact surface between the projectile and the debris flow deposit decreases, the resistance reduces, the acceleration decreases, the speed slows down relatively, and the curvature of the displacement curve decreases as the speed decreases.