When the elastic wave propagates in the special graded structure, the spatial frequency division and the wave energy concentration will emerge, which is called the rainbow trapping effect of elastic wave. Research on rainbow trapping effect will contribute to the advance of mechanical condition monitoring, vibration propagation and energy recovery. In this research, a novel triangular graded metamaterial beam (TGMB) is proposed to research on the flexural wave rainbow trapping effect and multiband vibration energy harvesting. Firstly, band structures of unit cells on the TGMB can be obtain from the finite element analysis, and the mechanism of the rainbow trapping of flexural waves is studied according to the band structures. Secondly, the spatial frequency division and the wave energy enhancement will be analyzed and validated by frequency domain and time domain finite element simulations on TGMB. Finally, an energy harvesting prototype was fabricated by pasting piezoelectric patches onto the TGMB, the multiband energy harvesting of the graded structure and the influence of incident wave frequency have been researched by the experiments. The results illustrate that the energy harvester based on TGMB can collect electrical power on three cut-off frequencies effectively.