Abstract: Installed on the roof of the electric multiple unit, the compacted layout inside the integrated high voltage apparatus box may cause the concentrated distribution of the electric fields on the fittings of some electrical equipment and lead to corona discharge. In this paper, a three-dimensional finite-element (FE) model of the high voltage apparatus box is established. Under the normal operating voltage, the overall electric field distribution in the high voltage box is analyzed based on the electrostatic field and under the abnormal condition, e.g. standard lightning impulse voltage, the overall electric field distribution and its variation with time in the high voltage box are analyzed based on the transient electric field. For the components with high surface electric field intensity in the high voltage apparatus box, different approaches have been proposed to adjust their electric fields. The surface electric field of the high voltage disconnector is adjusted by changing the shape of electrode; that of the bus-bar connected to the voltage transformer by laying the insulation sheath; still that of the upper and lower blades of the vacuum breaker by installing open corona rings, whose optimum parameters are determined through the orthogonal test method. Finally, the accuracy of the FE calculation is validated by experiments on the potential measurement of floating fittings. The presented FE calculation and optimization methods can provide certain reference for the engineering design of the high voltage apparatus box.