A system and a method for computing the relative rotation and the relative translation of back-to-back cameras comprise: a fixed mode and an adjustment mode. In the fixed mode, a reversible external camera is installed next to the back-to-back cameras, the external camera is flipped to shoot a calibration pattern from different positions and angles, the intrinsic and extrinsic parameters of each camera are computed, and then a reverse calculation is performed on the extrinsic parameters, as such the amount of translation and rotation of the back-to-back cameras are obtained. When the relative angle of the back-to-back cameras changes, the angle is automatically detected and the extrinsic parameters of the camera computed at the fixed mode is modified, and then the reverse calculation is performed on the extrinsic parameters to adjust the amount of translation and rotation of the back-to-back cameras.