The National Metrology Institute (NMI) of The Netherlands (VSL) developed a new 1200-kV reference measuring system for on-site calibration of lightning impulse (LI) voltage measuring systems. Improvement of the low-voltage grounding loop and triaxial signal transmission reduced oscillation ripples and enhanced the shielding of environmental high-frequency interference. Calibration and measurement capabilities are realized in the test voltage (U_t), front time (T₁), and time to half-value (T₂) with uncertainties of 0.7%, 3.0%, and 1.5%, respectively, up to 600 kV and 1.2%, 5.0%, and 3.0%, respectively, up to 1200 kV for direct comparison calibration. With additional linearity experiments, VSL is now able to provide LI voltage calibrations on-site up to 3000 kV. The VSL system was used as the reference for calibration and tuning of a 4000-kV LI divider to improve its accuracy performance. A measurement campaign was held to compare different LI measuring systems and research the linearity of the measuring system up to 3000 kV. The comparison results confirmed the good performance of the VSL 1200-kV reference measuring system and also indicated that the 4000-kV measuring system was improved after calibration and tuning with the VSL reference system.