Since the service is controlled by a GUI, the "UseSynchronizationContext" attribute was needed to solve the problem:
[ServiceBehavior(InstanceContextMode=InstanceContextMode.PerCall,ConcurrencyMode=ConcurrencyMode.Multiple,UseSynchronizationContext=false)]
现在开发很多WCf程序已经不托管在IIS下，而是自托管了，所以在客户访问服务时，如果不进行如上设置 ，无法实现真正多的多线程，造成访问排队。