A transmitting power level control method and system are provided, whereby the power of a terminal device in a wireless sensor network is saved and the lifetime thereof is extended. The control method includes the steps of: (A) broadcasting a plurality of transmitting power level (TPL) messages, each of which represents a respective TPL and the respective TPLs are different from one another, wherein each of the plurality of TPL messages is broadcasted at the respective TPL thereof; and (B) setting a TPL for the terminal device at a minimum one of the TPLs represented by the TPL messages received by the terminal device.