The electro-optical coefficient is an important parameter of lithium niobate crystal, which is closely related to the design and application of electro-optical devices. However, the synthesis conditions of lithium niobate crystals are different and this results in different electro-optical coefficients of each crystal, which brings difficulties to the error analysis of electro-optical coefficients. From the perspective of numerical analysis, Python, the open source package is used to fit the measured data. The specific value of fitting parameters and their uncertainties are obtained, and then the electro-optical coefficient of crystal is derived from the error transfer theory. Numerical analysis shows that the half-wave voltage method has high accuracy in measuring electro-optic coefficient, and its relative error is less than 2%. The fitting code of the widely used program Python is also given in this paper, and relevant researchers are welcome to use for reference.