Electro-optic and voltage-controlled diffractive properties of Fe-doped potassium sodium tantalate niobate (FeKNTN) crystal are presented. This crystal is grown by the top seeded solution growth (TSSG) method and the Curie temperature is 15 ℃. Effective quadratic electro-optic (QEO) coefficient Reff is measured with the one-beam ellipsometric setup, and its Reff achieves as high as 1.05×10-15 m2/V2 near the Curie temperature which is about 19 times that of LiNbO3 under an electric field of 500 V/mm. Reff declines with the increasing temperature. The voltage-controlled diffractive property is measured by a two-wave mixing setup. When the external electric field increases from 0 to 900 V/mm, the diffraction efficiency increases firstly and decreases afterward, and reaches the maximum value of 80% at 700 V/mm. These results show that FeKNTN is a promising electro-optic crystal and voltage-controlled diffractive crystal.