Linearity is one of the key technical parameters of the impulse voltage divider. With the rising of the rated voltage, the impulse voltage divider which can’t be calibrated over the full assigned measurement range must have a separate linearity test. In this paper, two methods of measuring linearity are introduced. The change in the ratio of output voltage to charging voltage of impulse voltage generator is measured. The measured voltage divider is compared with the spherical field measuring instrument or the wideband capacitive divider which the compressed gas standard capacitor as the HV arm. The effects of accuracy of charging voltage, charging time, compensation of charging voltage, inhomogeneity of charging voltage on the output voltage of impulse voltage generator have been studied. The output voltage of spherical field measuring instrument is proportion to the measured electric field intensity. The impulse voltage waveforms can be measured on both uniform filed and non-uniform field. The non-linearity of spherical field measuring instrument is below ±1%. At last, the above mentioned methods are used to measure the non-linearity of the same 1800kV impulse voltage divider. The results are in good agreement, and the spherical electric field measuring instrument can be used to calibrate non-linearity of UHV impulse voltage divider on-site.