The spectral characteristics of three violet InGaN/GaN single quantum well samples with the same average In composition but different In composition distributions are studied in this paper. By analyzing the electroluminescence spectra, energy band structures, wave function overlap and carrier concentration distribution, it is found that the single quantum well sample with linearly increased In composition along the growth direction has the highest luminescence efficiency, while the sample with linearly decreased In composition has the lowest luminescence efficiency. It is considered that the linear increase of the In composition can weaken the influence of the polarization field on the valence band, making the valence band in InGaN well smoother, which not only reduces the height of the hole injection barrier, but also increases the hole concentration in InGaN well. The overlap integral of the electron-hole wave function in the well is also enhanced, and the probability of radiative recombination is improved, so the luminescence efficiency of the InGaN quantum well with the linear increase of In composition is significantly increased.