In order to reduce drift caused by cavity variation,dispersion equalization of non-planar four-frequency differential laser gyro was investigated. Expression of DLG bias as a function of operating point and axial magnetic field was derived from classic theory of gas laser. Consequently the dispersion equalization was proved theoretically. Coil around gain tube was used to provide axial magnetic field with controllable strength for gain medium,and piezoelectric transducer attached to cavity mirror was used to adjust cavity length or operating point. Afterwards,experimental curve of bias versus cavity length and coil current was obtained. The results showed that bias sensitivity to cavity-length change was a function of axial magnetic field,which was zero with Zeeman splitting of gain curve equal to nonreciprocal splitting. Dispersion equalization reduces drift caused by cavity variation,so it helps to improve DLG precision.