To analyze the torsional random excitation of tracked vehicle powertrain system, the road surface excitation was transferred from frequency region to time region based on given the statistical characteristics of road roughness. The root of torsional random excitation was investigated and theoretic calculating formula was proposed. Then the dynamics model of tracked vehicle was constructed under different road surface by abstracting each element of the vehicle mathematically. The torsional random excitation spectrum was numerically calculated. The effects of road roughness, vehicle velocity and pretension having on torsional random excitation were deeply studied. The investigated results indicated that the road roughness and tracked vehicle speed were the main factors to determine the value of torsional random excitation.