Dual-frequency laser resulting from fine detuning twisted-mode cavity is discussed in this paper. Two measurement systems, a Fabry-Perot etalon and a confocal spherical interferometer, are adopted. The former reflects directly the interferogram of multimode and single mode. The latter can judge accurately the mode spectrum of single frequency and dual-frequency laser. Moreover, the experimental results provide the relationship between frequency difference and the angle of the relative rotation of the two quarter-wave plates and agree well with the theoretical analyses. The range of difference frequency is from 50 MHz to 1 GHz.