<p>Multi-span beams are challenging in two issues, including their complex behavior and their high vulnerability to train-induced vibrations. To deal with these issues, the use of inclined fluid viscous dampers (FVDs) has been proposed, which have uncomplicated and predictable performance. The objective of this research is to investigate the efficiency of FVDs on vibration reduction of multi-span continuous beams with unequal spans and under the passage of high-speed trains. To do so, the dynamic equation of the system is derived, solved using the Newmark Beta method, and the responses are compared with the results of the finite element method (FEM). Then, the system response without consideration of external dampers is obtained, and the critical conditions are determined. Then, several values for the damping coefficient, installation angle, and the horizontal distance of each damper from the nearest abutment are selected, and the effect of each parameter on the acceleration in the critical condition is investigated. In addition, to validate the obtained results, the particle swarm optimization (PSO) algorithm is employed, which provides the same results. The results show that inclined FVDs with suitable parameters can decrease the oscillations more than 50 percent. In addition to their efficiency, these dampers have simple performance, which makes their application generalizable to similar structures. Therefore, they are practical and highly recommended for retrofitting bridges.</p>