As an outstanding quality multi-functional crystal, potassium tantalate niobate (KTN) crystal is notable for its quadratic electro-optical effect. An electronically controlled birefringence effect based on the crystal is found during the experiment of electro-optical modulation. To illustrate this kind of performance that one electronically controlled beam is separated into two beams of orthogonal polarization light, the principle of quadratic electro-optical effect and the Fresnel equation are used to construct the mathematic model of beam separation deflection. The reason for the phenomenon is elucidated theoretically, and the function relationship between voltage, incident angle and deflection angle of extraordinary light is given. In the experiments, the electronically controlled birefringence phenomenon of KTN crystal is further measured and the corresponding theory is verified. The experimental results show that KTN crystal has obvious electronically controlled birefringence effect and large deflection angle of extraordinary in the case of 532 nm laser oblique incident.