The improvement of piezoelectric properties is always a key issue in the study of piezoelectric materials. The construction of composition gradient (CG) is a novel method to improve piezoelectric properties. However, the physical mechanism of CG influence on the piezoelectric properties has to be analyzed further. We investigate the origin of the CG-affected piezoelectric property using a manganese-doped potassium sodium niobate single crystal (that is Mn: KNN) as a model system. The relationship between CG, microstructure and spontaneous polarization was explored by designing and growing Mn:KNN single crystals with different CGs. It is found that the built-in electric field (E_in) in single crystals is constructed by introducing the CG in the crystals. The large E_in induces a more ordered, large-sized striated domain structure. The Mn: KNN single crystal with a large CG has a uniform orientation of spontaneous polarization and large macro-piezoelectric properties (83 pC N⁻¹). Therefore, it is important to explore the relationship between CG and the piezoelectric properties of single crystals in lead-free piezoelectric materials. The results can be used to further develop high-performance lead-free piezoelectric materials.