In Xizhe HVDC project the PROFIBUS DP communication false alarm occurs frequently,which is triggered by the detection of the heartbeat signal.It is found that the PROFIBUS DP communication system is a multi-link asynchronous system.The time window of the heartbeat signal is changed by the data transfer in the multi-link asynchronous system,and the continuous multi-frame has the same heartbeat signal,which causes the false alarm of PROFIBUS DP.In this paper,the characteristics of heartbeat signal transmission in multi-link asynchronous system are analyzed in detail.The research method can be applied to similar structure commnunication systems.