A series of experiments are carried out by modulating piezo voltage of an external-cavity continuously tunalbe diode laser. Cesium atom D2 line (6S1/2→6P3/2) absorption spectrum is measured by harmonic detection with a lock-in amplifier. The results about various harmonic signals in experiment are agreement with that in theory. The high-order harmonic detection may result in a significant improvement in signal-to-noise-ratio (SNR).