Optical waveguide amplifiers are essential devices in integrated optical systems. Their gain bandwidths directly determine the operating wavelength of optical circuits. Due to the difficulty of developing wideband gain media, it has been a challenge to fabricate devices with broadband amplification capability, resulting in few reports on multi-band polymer waveguide amplifiers. Here, a polymer waveguide amplifier is demonstrated, which achieves loss compensation covering the whole (S+C) band by using NaYF4:Tm,Yb@NaYF4@NaYF4:Er nanoparticles (NPs)- doped SU-8 as the gain medium. The NPs with a layer-doped core-multishell structure not only provided two emitters required for (S+C)-band amplification, but also reduced the energy transfer (ET) between them. Under 980-nm excitation, the full width at half maximum (FWHM) of the emission peak of NPs reached 119 nm, and the relative gain in the (S+C) band was about 6-8 dB, successfully expanding the operating wavelength from single-band to multi-band.