The quality of indoor environment has close relationship with peoples health, while people spend most time indoor during their life time. The development of relative technologies gives us a chance to monitor and change the indoor environment conveniently, such as sensor technology, computer technology and wireless communication technology. WSN (Wireless Sensor Network) has been widely used in many areas and especially in environment monitoring applications. This paper proposes a monitoring system applied in indoor environment which can provide an access to four kinds of parameters, including temperature, humidity, formaldehyde concentration and light intensity. The system is an application of WSN based on TinyOS. The system presented in this thesis is comprised of an upper PC, a sink node and a few sensor nodes. In addition, a monitoring platform with friendly human-machine interface is designed on the upper PC. The result of system test shows that this system can monitor the environmental parameters indoor sequentially and reliably.