Based on the frequency properties of laser, a novel method for measuring modes characteristic of laser gyro resonator is presented to confirm the mode characteristics of of measured laser. In which the characteristic distribution of laser modes are measured on-line by using a scanning interferometer with a con-focal resonator, take the known frequency spacing and swept rate of scanning interferometer as scale, it translates unfathomable frequency spacing into fathomable time spacing, and calculate the inverse proportion of different transverse mode frequency spacing and longitudinal mode frequency spacing. Besides it compares this value to the theoretic inverse proportion of the laser gyro, thereby the accurate mode characteristic of laser gyro resonator comes true. The experimental results of some laser gyro shows the relationship between the modes characteristic and its discharge current: the oscillating transverse mode in ring gyros cavity gradually changes from sigle fundamental mode to mixed mode with fundamental mode and high order mode. And the accurate mode characteristics is gained, the error between experimental results and theoretic value is less than 2%,The measurement accuracy and repetition of transverse mode frequency spacing and can be up to 1 MHz and 0.4 MHz respectively.