Letting the beam doubly pass an acousto optic frequency shifting system and using the technique of saturated absorption spectra of cesium atoms we realized the frequency offset locking of a single longitudinal mode laser diode to the hyperfine transition of 6 2S 1/2 F=4→6 2P 3/2 F′=5 of cesium atoms. The characteristic is that the detuning can be changed conveniently by tuning the working frequency of the acousto optic frequency shifter without readjusting the optical system.