The invention provides a calibration system and method for a field mill without being covered with a metal plate. The calibration system comprises a high-voltage DC power supply, a high-voltage DC voltage measuring meter and two parallel pole plates, wherein the high-voltage DC power supply is used for generating high-voltage DC voltage; the high-voltage DC voltage measuring meter is used for measuring a voltage value of the high-voltage DC voltage generated by the high-voltage DC power supply; and the two parallel pole plates are used for generating a high-voltage DC electric field when the high-voltage DC power supply generates the high-voltage DC voltage. According to the method, the high-voltage DC electric field is generated between the two pole plates through applying the high-voltage DC voltage, and calibration is performed on the field mill through considering the distortion of the DC electric field. The field mill without being covered with the metal plate is calibrated to beequivalent to a field mill covered with a metal plate with the size being not less than 1m*1m in field intensity, so that the measurement accuracy is improved. The field mill without being covered with the metal plate is adopted to perform direct measurement when being applied to measuring the DC combined field intensity of a practical DC project, so that the labor intensity is greatly reduced, and the measurement efficiency is improved.