This paper introduce the technological process and structures of fan—out wafer level packaging. The characteristic impedance of coplanar waveguide for fan out packaging is analyzed. In view of impedance discontinuity of coplanar waveguide in the cross—regional, the influence of the width of signal and ground line, the separation between ground line and signal line is analyzed by using ANASYS simulation. The influence upon impedance mismatch was analyzed in the time and frequency domains. The most important influencing parameter is adjusted to reach impedance matching of transmission line. The signal reflection noise before and after parameter adjustment is compared. The results show that the signal noise is reduced from 12% to 2% by adjusting the separation between ground line and signal line in fan—out region, thereby significantly reducing signal distortion.