Abstract-How to sense and monitor the environment with high quality is an important research subject in the Internet of Things (IOT). This paper deals with an important issue of the balance between the quality of target detection and lifetime in Wireless Sensor Networks (WSNs). Two target monitoring schemes are proposed. One is TDSFK (Target Detection with Sensing Frequency K) which distributes the sensing time that currently is only on a portion of the sensing period into the entire sensing period. That is, the sensing frequency increases from 1 to K. The other scheme is TDASF (Target Detection with Adjustable Sensing Frequency) which adjust the sensing frequency on those nodes which have residual energy. The simulation results show that the TDASF scheme can improve the network lifetime by more than 17.4%, and can reduce the weighted detection delay by more than 101.6%.