Taking the grid structures of the car body workshop of Shanghai Volkswagen’s Yizheng fifth factory as research object and using random vibration method,the dynamic responses of the grid structures were analysed under independent three orthogonal random seismic excitations.Analysis results indicate:the structure is characterized by high density vibration modes and uniform frequency varation,which may at first be twisted and destroyed under seismic excitation.Under horizontal random seismic excitations,a few bars at the bottom chord will fail and the draw press stress of No.1 cross section bar at the upper chord will be relatively great,though the whole structure is safe.Under random seismic excitations in x and y direction,the draw press stresses of No.1 cross section bars at upper bottom chord have a great difference,while the other section bars’ stresses are of little diversity between each other.