If the frequency of a laser is modulated sinusoidally, the first harmonic component of saturated absorption signal can be used as error signal to stabilize the frequency of the laser. We introduce the principle of using sampling integration technique to obtain the first harmonic signal and the realization of sampling integration and digital proportional-integral differential (PID) controller by means of a microprocessor. Compared to traditional lock-in amplifiers and analog PID controllers, the new technique is more reliable in performance, simpler in construction and easier to be realized. An external-cavity diode laser (ECDL) frequency stabilization system is built with this sampling integration based on Cs atom saturated absorption. Relative frequency fluctuation better than 1.2×10-9(p-p) can be achieved and the average locking duration is about 7 days. The system can also automatically scan the saturated absorption peaks, lock to the desired peak and resume locking when lock is broken by accident.