1. A method for measuring the interface state density, wherein, the method is applied by a conductance technique, the method comprises the steps of: biasing a MOS capacitor structure to be measured in an accumulation region, measuring the MOS capacitor structure under a fixed bias voltage and at predetermined scanning frequencies in the accumulation region by using a Gp-G model, and calculating the values of the series resistor in series with the MOS capacitor structure at respective predetermined scanning frequencies to obtain a series resistor model; obtaining an accurate model in an inversion region from the series resistor model varying with the predetermined scanning frequencies obtained in the accumulation region, obtaining the measurement results of interface state according to the accurate model.