Clarifying the electric field distribution characteristics of mechanical insulated rail joints in high-speed railway stations under working conditions is the key to solving the insulation failure problem. Based on the electrostatic field finite element method， the electric field distribution of the intact insulated rail joint under steady-state voltage and transient overvoltage is calculated considering the thickness， the material of the insulated rail joint and the power supply mode， respectively， and the influence of different working conditions on the electric field distribution of the insulated rail joint is also analyzed. The possible defects of the insulated rail joint are analyzed. The effect of bubbles， air gap and carbonization on the electric field distribution of the insulated rail joint is studied. The results show that the overall electric field distribution of the insulated rail joint is uneven. Partial discharges may occur at the top surface of the insulated rail joint under transient overvoltage. The thickness and the power supply mode have a certain effect on the electric field distribution of the insulated rail joint， and the effect of direct supply is larger than that of auto-transformer （AT） power supply mode. The influence of bubbles on the electric field distribution is related to their locations， and air breakdown is prone to occur around bubbles， causing partial discharge. Carbonization has a greater effect on the electric field distribution of the insulated rail joint. The carbonization depth changes the maximum value of the local electric field distribution intensity of the insulated rail joint， and the influence of carbonization depth on the electric field intensity is greater in the top region than in the waist and bottom areas.