In order to improve the frequency stability of laser in 1.5μm regions, two acousto-optic modulators were used to shift the laser frequency and the absorption spectra of an acetylene transition (12C2H2 P9) at 1530.37nm was used as the frequency reference. Then, the dispersion-like frequency discriminating curve was obtained by subtracting the two shifted absorption spectra of acetylene. The new method could avoid the extra frequency noise due to indirect frequency modulation of laser source in a conventional scheme. After locking laser, the typical frequency jitter was less than 5.8MHz in 50s, which is significantly better than the frequency fluctuation under free-running condition. Hence, the frequency stabilization of external cavity diode laser with a modulation-free output can be realized.