The effects of KNbO3 addition on the temperature stability of dielectric properties and energy-storage performances of Sr0.3Na0.25Bi0.25TiO3-MgO ((1-x)SNBTM5-xKN) ceramics were systematically investigated. It shows that KNbO3 has completely diffused into SNBT lattice to form a solid solutions. Two obvious maxima in permittivity curves were observed due to the formation of the polar nanoregions. For the sample with x = 0.05 addition, its permittivity variation region, where Delta epsilon'/epsilon'(25 degrees C) is no more than +/- 15%, retains wide the range from -55 degrees C to 200 degrees C, indicating that the ceramics can meet the requirement of X9R specification. Furthermore, this sample shows a high electric breakdown strength of 17.85 kV/mm and a high discharge energy density of 2 J/cm(3). The noticeable reduced remnant polarization induced by KNbO3 addition also contributes to the improved energy-storage performances. Thus, these novel ceramics with the aforementioned characteristics will potentially benefit the applications of high pulse power capacitors at high temperatures.