During the flight of relay satellite around Earth-Moon L2 libration point,there will be shadows caused by Earth and Moon. Some of the shadow durations are very long. Satellite platform security would be threatened when relay satellite in the long shadow period. Based on the engineering objective of lunar far side soft landing exploration and the halo orbit features of the relay satellite around the Earth-Moon L2 libration point,long shadows characteristics are analyzed,such as the distributions,and the correlations of shadow durations to halo orbit phase,amplitude,and configuration. Long shadow avoidance strategies and applicable scopes are discussed. Research results provide analysis method and solutions to long shadows of relay satellite around Earth-Moon L2 libration point.