The horizontal swing of the flat plate hanged by two lines with equal length is a complex problem.We studied the small angular swing of the flat plate in horizontal direction, which belongs to harmonicvibration.Second,the swing frequency of the flat plate is measured by smartphone installed “ phyphox”softwareand its light sensor.We changed the mass of the iron ball placed in the center of the plate and measured itsswing cycle of the system.The measured data were obtainde and the period curves are plotted by Origin softwareto calculate the swing period of i.lt is found that the experimental results are in good agreement with thetheoretical ones.This work provides an experimental surport for dealing with the oscillations of complex systems