To overcome the disadvantages of traditional Pound-Drever-Hall (PDH) laser frequency stabilization method, we have design a PDH laser frequency stabilization system based on orthogonal demodulation method.The system uses the same direct digital synthesizer (DDS) to synchronously generate three sine signals with the same frequency. One signal is used as local oscillator signal to drive an electro-optic modulator so as to produce the phase sidebands, and the other two sine signals with a phase difference of 90° are used as demodulation references. The in-phase and orthogonal components of the error signal are obtained by use of two analog demodulators, and the error signal of the frequency stabilization system is obtained when both components are digitally sampled and then operated by an algorithm of digital phase-sensitive detection. The key techniques for the PDH laser frequency stabilization based on the orthogonal demodulation method have been investigated, and an experimental system of laser frequency tracking has been established. Experimental results show that the F-P cavity can track the laser frequency variation in real time, with a tracking time of approximately about 1 hour.