In order to obtain the number of mobile devices WiFi on in some region, a WiFi device sensing system based on MT7620A chipset and embedded operation system OpenWrt was designed. The principle how a mobile device was studied, and then suitable hardware and software platform were chosen. After that, the system was set to monitor mode. With the help of sniff pro, probe requests from mobile devices were captured and media access control addresses were parsed. Implementation mechanism and key technology of the sensing system were elaborated, which include the creation of swap partition on a U disk and the algorithm of how related data captured and saved. The experiment result shows that our system can capture Probe Request stably and is ease to use, and it has some commercial value.