Suppression of stress and crack generation during picosecond laser processing in transparent brittle materials such as glass was successfully demonstrated by a picosecond laser pulse with temporal energy modulation. The origin of deterioration in processing accuracy could be interpreted in terms of the discontinuous movement of plasma in the vicinity of the focus. To reveal the effectiveness of the temporal energy modulation for smooth machining, such plasma motion was simulated by the finite-difference time-domain method. Furthermore, photoinduced birefringence was observed using a high-speed polarization camera.