The invention relates to an automatic awakening circuit applied to a wireless communication circuit in a cardiac pacemaker. An in-vitro program controller and an in-vivo cardiac pacemaker perform data transmission in a carrier-free base band digital signal mode through coupling of an inductance coil, and functions of an in-vivo cardiac pacemaker communication circuit except an awakening function are closed when data transmission of the program controller and the cardiac pacemaker is not performed. When the in-vivo cardiac pacemaker communication circuit is to be awakened, a working mode of the awakening function of the in-vivo cardiac pacemaker communication circuit is discontinuous work, i.e., 32-ms opening within 1s, when the in-vitro program controller needs to perform data interaction with the in-vivo cardiac pacemaker, the in-vivo cardiac pacemaker communication circuit is awakened through continuous sending of a specific awakening pulse group, once awakened successfully, the communication circuit is turned on and switched to a continuous working mode. Thus the in-vivo cardiac pacemaker communication circuit can be turned on through control of the in-vitro program controller, the working modes can be switched, and false awakening of the communication circuit by an interference signal can also be prevented.