In galvanic coupling conductive intracardiac communication(GCCIC) of the leadless pacemaker, the electrical signal transmitted directly through the myocardium and blood is inevitably affected by the cardiac cycle. The existed research paid more attention to the effect of the myocardium during communication. However, experiments show the change of blood volume also has a great impact on signal transmission. In this paper, the cardiac cycle is divided into seven phases with different blood volumes periodically, and the signal gain is monitored through a homemade dynamic measurement platform with two peristaltic pumps and several silicone tubes. Through in vitro experiment of the porcine heart, the changes of two channel gains caused by the cardiac cycle are analyzed. The results show that channel gains change periodically with the change of blood volume in the cardiac cycle, and the change trends of the two channels are different because the signal paths in the two channels are different. For the Right Ventricle - Right Atrium channel, gain varies from -62 to -47 dB and channel gain is inversely correlated with blood volume. For the Right Ventricle - Left Ventricle channel, when blood volume is less than 80 mL, channel gain is about -51 dB. And when blood volume is greater than 80 mL, channel gain fluctuates around -48 dB. This study shows the gain of the intracardiac communication channel is affected by the change of blood volume in the cardiac cycle.