To overcome the weakness of traditional detection approaches for regional staff density, and meanwhile obtain the regional staff density information better from Probe Request (PR) frame sended from mobile phones with the enabled Wireless-Fidelity (Wi-Fi), a detection probability model of regional staff density based on Wi-Fi devices was proposed. Firstly, the average PR frame time intervals of common mobile phones were obtained by experiment, which could provide a guidance for setting some parameters of probability model. Secondly, according to the IEEE 802.11 standard and Wi-Fi channel attributes, a Wi-Fi detector's mathematical model was built. Finally, reasonable values were assigned to these parameters on the basis of specific environment, and the detection probability of detector was simulated. The theoretical analysis and simulation results show that the proposed mathematical model can reflect the detection situations for staff density.