Transmission using excess power not only shortens the lifetime of sensor nodes, but also introduces interference. Packets should ideally be transmitted with moderate power. This work proposes a multi-level power adjustment (MLPA) mechanism for a wireless sensor network to prolong the lifetime of individual nodes and the overall network. An analytical model of the MLPA mechanism with m distinct power levels (m-LPA) is constructed. For 3-LPA, the closed-form expression of the optimal power setting is determined and the mean transmission power is minimized to one third of original fixed power. Besides, we found that average power consumption of our proposed mechanism is 47% higher than original. Thus, each node can extend 2.5 times of life time. And we shown the relations between m and density in simulation result.