This paper presents a time-variant coverage mechanism, proactive coverage. In proactive coverage, all sensor nodes work in lower power surveillance manner which can detect target intrusion with high probability to save energy when the target flow doesn’t arrive. As soon as the target flow arrives, sensor nodes are awakened to build a local high quality coverage networks to sense intervening targets. When target flow leaves the target field, sensor nodes converge to a quiet surveillance state. Proactive coverage is more energy efficient than static area coverage, higher sensing quality than event-driven coverage. This paper analyzes preliminary problems in proactive coverage and finds theoretic results on initial detecting delay, awaking nodes strategy and active sensing duration. Numeric results from simulation reveal that initial detecting delay in proactive coverage is trivial. Compared with static area coverage, this coverage mechanism for an adequate scale target flow (above 30 targets) can prolong the network’s lifetime to near 4～7 times.