Absorption length has an influence on signal to noise ratio (SNR) and slope of modulation transfer spectra. An equation of modulation transfer spectroscopy was derived by dividing absorption length into n units and integrating signal of every unit. Theoretically the relative signal intensity and central slope of spectrum with different absorption length were calculated according to the equation, from which optimal absorption length was obtained. Finally both SNR and slope of modulation transfer spectra of I2 with different absorption length were compared experimentally while I2 cell temperature was held at -15 ℃. The results of comparison showed that modulation transfer spectra was optimal at three-pass (120 cm). The frequency stability of 9×10-14 at 1 s integration time at three-pass was estimated. So it is promising for improvement of laser frequency stability when modulation transfer spectra with optimal absorption length are used in laser frequency control.