The roller beam is one of the main loadbearing members of parachute locks. In the rocketsled tests of a parachute lock, the roller beam failure led to fail in opening the drag chute. According to the failure analysis, the mode of roller beam failure under impact is shear failure. In the paper, the roller beam impact shear failure mechanism was analyzed and it is shown that the parachute lock obtains an initial impact velocity under the action of the initial impulse of extraction parachute, and then collides with the roller beam, which causes the roller beam shear failure. The initial impact velocity and the equivalent mass of the motion mechanism were obtained by modelling the dynamics of the parachute lock, and the roller beam was modelled as a circular crosssection beam clamped at both ends under uniform impact load. The rigidplastic material model was used, and the thickness coefficient of the shear failure and the bendingshear interaction were taken into account. The threshold impact velocity of roller beam failure was derived based on the bendingshear theory. The effect of the mass ratio of the sleeve to the roller beam on the roller beam failure was discussed. Some improvements for structure design were proposed.