A high frequency-stable laser source for Brillouin distributed optical fiber sensor (DOFS) is reported. A distributed-feedback (DFB) laser is used as the maser laser for optical fiber sensor measurement. Another DFB laser is used as the slave laser of which the frequency is tracked with the master laser. Thus the frequency difference of the two lasers can be locked at 11 GHz. With this source for distributed optical fiber sensing applications, the frequency of the beat frequency signal is only several million hertz. As a result, the noise can be decreased and the cost of the equipment will be reduced. The results indicate that the range of frequency is ±1 MHz.