There are some inevitable problems such as vibration, instable connection between the optical elements, hot expansion and cold contraction in the practical application of optical voltage transducer (OVT) based on Pockels effect. These factors cause the deviation between the optical elements, which affects the internal electric field distribution in the electro-optic crystal. The OVT based on convergent polarized light interference was adopted as an example. The simulation analysis and experimental research show that when the incident light has a ±0.5° deviation or the BGO crystal has a ±1° deviation, it leads to electric field integral errors of about 0.107% or 0.124%, respectively. As the OVT needs to guarantee the accuracy of 0.2%, the effect induced by the electric field integral errors cannot be ignored. Therefore, a new method of medium enwrapping was proposed and the alumina ceramic was selected as the medium to wrap out of the electro-optic crystal. So that the maximum electric field integral errors can be reduced to 0.001% or 0.003% respectively. The experimental result and its applications show that the medium enwrapping method is simple, practical and effective.