Due to the deep integration of information technology and operational technology, networked control systems are experiencing an increasing risk of international cyber attacks. In practice, industrial cyber security is a significant topic because current networked control systems are supporting various critical infrastructures to offer vital utility services. By comparing with traditional IT systems, this paper first analyzes the uncontrollable cyber threats and classified attack characteristics, and elaborates the intrinsic vulnerabilities in current networked control systems and novel security challenges in future Industrial Internet. After that, in order to overcome partial vulnerabilities, this paper presents a few representative security mechanisms which have been successfully applied in today's industrial control systems, and these mechanisms originally improve traditional IT defense technologies from the perspective of industrial availability. Finally, several popular security viewpoints, adequately covering the needs of industrial network structures and service characteristics, are proposed to combine with burgeoning industrial information technologies. We target to provide some helpful security guidelines for both academia and industry, and hope that our insights can further promote in-depth development of industrial cyber security.