The third harmonic generator is an important part of the ICF laser facility. It is necessary to adjust the KDP crystal alignment offline before it works efficiently online. The basic process of the adjusting is to measure the harmonic generation efficiency at different detuned angles by scanning the crystal, then fit the dates with multinomial function, and draw a rocking curve and decide the best angle for the harmonic generation. In this paper, a precision and automatic crystal alignment adjusting system is designed base on the theory analysis. In the experiment, the system can adjust the crystal alignment automatically, and it has an accuracy of 30 μrad in phase matching angle measurement, and 4% in conversion energy measurement.