Artificial intelligence (AI) has profoundly changed and will continue to change our lives. AI is being applied in more and more fields and scenarios such as autonomous driving, medical care, media, finance, industrial robots, and internet services. The widespread application of AI and its deep integration with the economy and society have improved efficiency and produced benefits. At the same time, it will inevitably impact the existing social order and raise ethical concerns. Ethical issues, such as privacy leakage, discrimination, unemployment, and security risks, brought about by AI systems have caused great trouble to people. Therefore, AI ethics, which is a field related to the study of ethical issues in AI, has become not only an important research topic in academia, but also an important topic of common concern for individuals, organizations, countries, and society. This paper will give a comprehensive overview of this field by summarizing and analyzing the ethical risks and issues raised by AI, ethical guidelines and principles issued by different organizations, approaches for addressing ethical issues in AI, methods for evaluating the ethics of AI. Additionally, challenges in implementing ethics in AI and some future perspectives are pointed out. We hope our work will provide a systematic and comprehensive overview of AI ethics for researchers and practitioners in this field, especially the beginners of this research discipline.Impact Statement-AI ethics is an important emerging topic among academia, industry, government, society, and individuals. In the past decades, many efforts have been made to study the ethical issues in AI. This article offers a comprehensive overview of the AI ethics field, including the summary and analysis of AI ethical issues, ethical guidelines and principles, approaches to address AI ethical issues, and methods to evaluate the ethics of AI. Additionally, some challenges and future perspectives are discussed. This article will help researchers to obtain sufficient background and a bird's eye view of AI ethics, and thus facilitate their further investigation and research.