The invention provides a capacitance sensor calibration system for measuring very fast transient overvoltage (VFTO), which comprises a high voltage steep pulse generator, a conical transmission cylinder, a capacitance sensor, a resistance voltage divider, coaxial cables, a first oscilloscope and a second oscilloscope, wherein the high voltage steep pulse generator is used for generating high voltage steep pulse; the high voltage steep pulse is transmitted to the capacitance sensor and the resistance voltage divider which are arranged in a gas insulated metal-enclosed switchgear (GIS) by the conical transmission cylinder; the conical transmission cylinder is coaxially connected with the GIS; the resistance voltage divider transmits a resistance partial voltage signal to the first oscilloscope by means of the coaxial cable; and the capacitance sensor transmits the capacitance partial voltage signal to the second oscilloscope by means of the coaxial cable. The calibration system takes the high voltage steep pulse generator as a signal source so as to better accord with the actual waveform of the VFTO; and in addition, the capacitance sensor and the resistance voltage divider are arranged on the cylinder of the GIS, thus the calibration system can calibrate the capacitance sensor under the approximately actual installation condition.