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 °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. A material concept of electrically enhanced diffused polymorphic phase transition (EED‐PPT) is developed to resolve the long‐standing issue of temperature‐sensitivity in lead‐free (K,Na)NbO3 piezoelectrics. Experimental and theoretical studies reveal that EED‐PPT can remarkbaly boost the temperature stability of (K,Na)NbO3, where the normalized strain d33* almost keeps constant from room temperature up to 140 °C.