The indicators of perception gap analysis on digital preservation includes the preservation reason, importance, threats, financial insurance, system performance and construction of institutional repositories. The indicators of its practical gap analysis include enforcing the preservation policy, collecting, processing, storing and accessing to digital resources, managing digital preservation and developing user services. The results of analyzing whole gap find that now research libraries have a middle overall gap on digital preservation, a low cognitive gap and a high practice gap. The results of condition gap analysis find that the gap is smaller, if more digital resources a research library preserves, more rational the allocation of human resources is and stronger the system's future expansion is. Finally this paper concludes that measures of reducing the gap include developing quality assurance standards of digital preservation, establishing its economic evaluation methods, reasonably equipping with the number and skills of employees, and building a well scalable digital preservation system.