Objective： To investigate the coronary sinus（CS） electrogram and the relationship of CS electrogram with local electrical activation of the coronary sinus muscle sleeve（CSMS）， and to demonstrate that A wave of CS electrogram was not only an atrial activating wave， but a complex wave of CSMS potential and left atrial（LA） potential. Methods：Forty-two patients with paroxysmal supraventricular tachycardia（PSVT） were involved. Under sinus rhythm， RS2 stimuli were issued at the proximal CS electrodes（CS9，10）， and then decreased by 5 ms steps. A wave morphology， changing of its order and pattern， double potential and activating time of CS were observed. Results：During sinus rhythm， A wave of distal CS（CS1， 2） in 67% patients showed firstly down and nextly up direction， with broad pattern and multiple compositions. RS2 firstly captured A wave in proximal CS， made it advance in time. Gradually with the decrease of RS2， A wave of CS in the middle segment also advanced， and its pattern changed into firstly down then up direction. When RS2 was early enough， A wave of distal CS（CS1，2） changed into first up then down direction. Conclusion： A wave of CS is not a pure atrial wave， but a potential fusion of CSMS and LA. During sinus rhythm， proximal and middle CS is activated mainly by the pulse through Bachmann bundle and CS， LA potential fuses with CSMS potential. The distal CS is activated by the pulse through Bachmann bundle. Under some conditions， two electric potentials could separate.