The invention discloses a device and method for measuring modal parameter of a cable-stayed bridge for normal traffic, comprising: selecting a reference measuring point for setting a sensor; selecting a plurality of measuring positions for each measuring segment of the bridge, each of which is used for recording data of measuring position sensors and the data of reference measuring position sensors; activating sensor each of measuring point by the taking the free traffic as vibration measuring source, and sequentially transmitting the collected signals to a data collector, a low-pass filter, a extended case, a computer; the computer is used for recording digital signal from the extended case; alternatively measuring the segment, repeating the measuring and signal transmitting process to the end; then using the computer to process the collected signal. The method according to the invention can identify modal parameter of the cable-stayed bridge after being used for a few years without influencing the traffic on the bridge, in order to provide a feasible method for vibration measurement.