The strong periodic surface electric strength caused by spontaneous polarization may be used as an undulator instead of Wiggler magnet to produce free electron laser. But the free electrons falling on it screen the potential gradually.Some screen-relif measures are proposed in this paper. Every positive polarized domain on the polarized crystal surface of LiNbO3 is covered with a piece of insulator film and covered further with an earthed conductor film to protect this polarized crystal surface from electric screening by free electrons. It is proved that the sum of induced charges on two surfaces of each part in these conductor films is zero. The free electrons falling on it will escape into the Earth at once.