When the voltage of the PZT on the back mirror of the ECDL is scanned, there exits a phase difference of the frequency variation and the background energy variation. It is realized that the absorption signal and background signal can be obtained separately at the same detector by use of the upper laser character and a lock in amplifier. A better 1f-4f harmonic signals can be obtained. If the phase difference doesn′t exit, the background harmonic signal cannot be eliminated. It is very important for setting up a miniature high sensitivity gas sensor.