Considering that improved accuracies have been achieved in observations, reference systems, models and time scales, and that the International Celestial Reference System (ICRS) has been introduced into use, the IAU General Assembly of 2000 resolved to adopt a new Celestial Intermediate Pole (CIP), a new Celestial Intermediate Origin (CIO), a new precession-nutation model, a new definition of UT1, and a new intermediate moving reference frame from 1 January 2003, thus it makes a great impact on astrometry. There are many great changes in contents and practices of astrometry. In this paper, the changes of terminology, concepts and definitions are described. The reasons for the changes, the influences on classical astrometry are discussed. A set of new theory and method of astrometry are being modified, we should catch up with development of this field.