We present a method of measuring the transmittance and Kerr coefficients of a paraelectric crystal by digital holography. Digital holograms of the crystal with different voltage are recorded, the amplitude and phase information are extracted from numerically reconstructed image. The transmittance of crystal is measured by use of the amplitude information, and the spatial distribution of the induced refractive index change can be obtained by different phase of different voltage and the effective electro-optic coefficient is calculated by means of the refractive index change. Experiment has been carried out by examining a paraelectric phase Mn0.25%:KLTN crystal, and the research results show that the method presented in this paper is successful and feasible, which are irrelevant at the shape of crystal and the size of light source.