The period formula of simple harmonic vibration and the law of coupled two-degree freedom vibration are explored by using the built-in sensor of smart phone and the APP of Phyphox, and the natural frequency of multi- degree freedom vibration system (pedestrian bridge) is also measured. The experiment results show that the period formula of simple harmonic vibration obtained by mobile phone and Phyphox is very close to the theoretical formula. For two-degree freedom system, it is verified that the idea of normal- model is feasible in the analysis of coupled vibration system. For the bridge such a complex multi-degree freedom vibration system, the use of mobile phones can be very convenient to obtain its natural frequency.