A power flow method is adopted to calculate the vibration velocity of track and bridge induced by wheel-rail combined roughness. A 2D acoustical model including track，bridge and sound field is used to obtain the mean square velocity and radiated sound pressure of the track and bridge produced by unit forces on the rail and bridge. According to the principle of vibration power equivalence，the actual sound pressure radiated from rail and bridge is determined by combining the two mentioned steps. The accuracy of the noise prediction method is validated by the measured results in field from a pure urban rail transit U-bridge. It is then used to compare the noise distribution characteristics from the pure rail transit bridge and double-deck viaduct composed of the upper urban road and the lower railway. The results show that：（1）the noise at the scalloped region above the upper bridge deck is obviously reduced by the upper road bridge，and the horizontal distance closer to the track centre，the greater the noise reduction；（2）the noise in the areas below the upper road bridge deck is increased by 3~10 dB without the noise barriers on the rail bridge，and the horizontal distance closer to the track centre，the greater the noise increment；（3）the noise barriers continue to reduce the noise in the scalloped region above the upper bridge deck and to increase the noise at the rest areas. These findings provide a beneficial reference to control the rail transit noise through the combination of noise barrier and the double-deck viaduct.