Mechanoluminescent (ML) materials have mechanical-light conversion properties and can generate luminescence under mechanical stress, which makes the ML materials have high application value in optical information display. In this work, the crystal structure and defect distribution are adjusted by changing the K⁺/Na⁺ ratio of the ferroelectric matrix K_xNa_1–xNbO₃∶0.5%Pr³⁺ (K_xNNOP), and the effects of K⁺ content on the photoluminescence (PL) and ML properties are systematically investigated. The research results indicate that as the K⁺ content increases, the symmetry of the crystal is enhanced, leading the PL intensity of the K_xNNOP samples to decrease. It is worth noting that the emission peaks caused by the ³P₁→³H₅ and ³P₀→³H₅ transition at the Pr³⁺ electron level appear in the PL spectra of the components with higher K⁺ content under the light excitation of 450 nm, which is attributed to the different energy level positions of the internal valence electron charge transfer states within Pr-O-Nb, caused by the change in the distance between Pr³⁺ and Nb⁵⁺. Under the compressive stress, the K_xNNOP (x = 0, 0.01, 0.02, 0.1) components exhibit the bright red ML, and the ML intensity increases with the K⁺ content increasing. The K_0.1NNOP component exhibits the highest ML intensity emission. In particular, the ML behavior has the characteristics of repeatability and recoverability. The trap energy levels in the K_xNNOP samples are investigated by thermoluminescence curves, revealing that the enhancement of ML in K_0.1NNOP may be related to the differences in trap density and trap depth, caused by changes in K⁺ content. Based on these results, a model is established to elucidate the possible ML mechanism in K_xNNOP.