The invention relates to insulation monitoring of a direct current system, and particularly relates to an instant alternating current invasion prevented insulation monitoring device. The device comprises a host, a wire selection device and a direct current leakage current sensor, wherein the host comprises a CPU data processing module, a data collection module, a balance bridge and unbalanced bridge circuit module, a power supply module, a display module, an alarm module and a communication module; the CPU data processing module is connected with the data collection module; the data collection module comprises a multi-channel collection electronic switch, an operational amplifier and a high-precision AD chip, and is used for collecting positive and negative earth grounded voltage of a direct current bus and voltage of the direct current bus in real time; the balanced bridge and unbalanced bridge circuit module comprises a group of balanced bridge resistor and a detection bridge resistor; the display module and the alarm module are connected with the CPU data processing module; and the wire selection device is connected with the CPU data processing module and the direct current leakage current sensor.