High　piezoelectricity　of　(K,Na)NbO3　(KNN)　lead-free　materials　benefits　from　a　polymorphic　phase　transition　(PPT)　around　room　temperature,　but　its　temperature　sensitivity　has　been　a　bottleneck　impeding　their　applications.　It　is　found　that　good　thermal　stability　can　be　achieved　in　CaZrO3-modified　KNN　lead-free　piezoceramics,　in　which　the　normalized　strain　d(33)*　almost　keeps　constant　from　room　temperature　up　to　140　degrees　C.　In　situ　synchrotron　X-ray　diffraction　experiments　combined　with　permitivity　measurements　disclose　the　occurrence　of　a　new　phase　transformation　under　an　electrical　field,　which　extends　the　transition　range　between　tetragonal　and　orthorhombic　phases.　It　is　revealed　that　such　an　electrically　enhanced　diffused　PPT　contributed　to　the　boosted　thermal　stability　of　KNN-based　lead-free　piezoceramics　with　high　piezoelectricity.　The　present　approach　based　on　phase　engineering　should　also　be　effective　in　endowing　other　lead-free　piezoelectrics　with　high　piezoelectricity　and　good　temperature　stability.