A new method for measuring cavity length of optical resonator is proposed by measuring the free spectral range of optical resonator and the intracavity resonance laser wavelength with narrow linewidth lasers as light sources. The measurement theory of optical resonator cavity length is strictly deduced, and the measurement conditions and results are analyzed and discussed by theoretical simulation. An optical resonator with cavity length about 100 mm is measured precisely with the developed narrow linewidth laser as light source, of which the linewidth is 1.9 Hz, the frequency instability is 1.7×10-14 s-1 and the central wavelength is 1550 nm. The free spectral region of the optical resonator is measured, and it is obtained that the cavity length of the optical resonator and the precision are 0.10024407 m and 22 nm, respectively. The intracavity resonance laser wavelength of the optical resonator is measured, and it is obtained that the cavity length of the optical resonator and the precision are 0.1002440884 m and 0.21 nm, respectively. The accuracy is improved by two orders of magnitude. The proposed method is expected to promote the development of basic physics research, physical properties precision measurement of materials, fiber sensing and so on.