The exact displacement solution of the dynamic equation of a simply-supported beam under moving load series is derived. Three dimensionless parameters are introduced to study the influence of load speed, load frequency and structural damping on the dynamic responses of the beam, and analyze the mechanism of resonance and vibration cancelation mechanism of simply-supported beam under moving load. The results show that the maximum displacement at the mid-span increases in a sinusoidal form with gradually increasing amplitude, instead of in a monotonous way. The residual displacement and velocity of the beam close to zero when the load series pass through at a vibration elimination speed. The resonance speed obtained by the calculation of the load interval, the resonance will be suppressed, when the resonance speed is also the vibration elimination speed. When the speed of harmonic load is low, the displacement of the beam will achieve maximum under condition of the load frequency equals to the first-order frequency of the beam; while the displacement of the beam will achieve maximum under condition of the load frequency not equals to the first-order frequency of the beam, if the speed of harmonic load is high.