(K,Na)NbO3 (KNN) is a promising lead-free ferroelectric/piezoelectric system, to which incorporating BaZrO3 can greatly enhance its piezoelectricity, but the mechanism is not clear. This work was conducted to investigate the phase transition in the BaZrO3-modifed KNN system and its contribution to piezoelectricity enhancement, using thin films with a fixed orientation and high compositional homogeneity fabricated by a sol-gel method. Two ferroelectric-to-ferroelectric phase transitions are revealed, which correspond to monoclinic M-C- M-A phase transition at higher temperature and rhombohedral-monoclinic M-C phase transition at lower temperature. It is difficult to distinguish these phases in KNN-based bulk materials, but their differences are clear when conducting high-resolution X-ray reciprocal space mapping (RSM) on the present thin films. Piezoresponse force microscopy experiments also revealed an interesting finding that local piezoelectricity of monoclinic phases was higher than that of rhombohedral ones in KNN-based thin films. This work could shed insights on the fundamental understandings for the effect of the chemical doping, and offer guidance for property optimization in the KNN-based lead-free piezoelectrics.