In order to simulate the phenomenon that electric shock defibrillation shortens action potential duration, the synchronous repolarization process is introduced into Luo-Rudy phase I model of cardiac tissue. Effects of synchronous repolarization on dynamics of spiral waves and spatiotemporal chaos are investigated. Numerical results show that when the control period is small, the synchronous repolarization can effectively eliminate spiral waves and spatiotemporal chaos. However, the synchronous repolarization can only eliminate spiral waves or spatiotemporal chaos under some control parameters. When spiral waves cannot be controlled, the transition from spiral wave to spatiotemporal chaos or the spiral wave with longer period and wavelength is observed. The control mechanism is analyzed.