Based on the empirical electron theory of solids and molecules, the valence-electron structures (VES) of sodium–potassium niobate ((KxNa1−x)NbO3) were calculated, and the relationship between the VES and the piezoelectric properties was determined. The results indicate that the piezoelectric constants are continuous and equal in the composition range of 0.4≤x≤0.5. The piezoelectric properties do not dramatically improved in the so-called morphotropic phase boundary (MPB). The experimental results show that the change trend of the piezoelectric constants is in good agreement with the calculation results. Contrary to traditional MPB, the so-called MPB of (KxNa1−x)NbO3 has no significant influence on the piezoelectric and ferroelectric properties. In other words, it was speculated that the existence of the traditional MPB was possibly controvertible in the KNN system.