There are two states in event-driven wireless sensor network, one is non-event state, and the other is event state. In non-event state, the network traffic is very small and the requirement for real-time performance of network is not strict. However, in event state the traffic is large, the critical information would be lost once the link quality goes bad without responding timely. This paper does research on warning mechanism of link quality in event state. And in event-driven wireless sensor network, there are two cases of packet loss, one is successive packet loss and the other is random packet loss. For the successive packet loss of link, this paper proposes a warning mechanism named SPLWM (successive packet loss warning mechanism). The key issue what SPLWM settle is finding out the warning threshold of successive packet loss. If the number of successive packet loss exceeds threshold, we consider there are some problems in the link, and warning messages should be reported. For random packet loss, this paper proposes a warning mechanism named RPLWM (random packet loss warning mechanism) which makes some improvements on conventional method. The key issue what RPLWM settle is doing warning to the case that the sum of packet loss in sample cycle exceeds the min tolerance threshold, it reacts more quickly than conventional method. With these warning mechanisms, we can do warning to the link in bad state. And with the captured warning messages, the upper-layer protocol can make analysis and estimation of link and then make some adjustments to link. With the mechanism mentioned above, the real-time performance and reliability of network will be improved.