Cloud computing is increasingly popular in recent years. As an application of cloud computing, cloud workflow is widely applied in scientific research and commercial field. And the cloud workflow scheduling problem is a major concern of the research on cloud workflow application. To study the state of the art of cloud workflow scheduling, some efforts are made and presented in this paper. Firstly, the retrieval is conducted to analyze the trend of the amount of related papers. Then, taxonomies are made from different aspects of cloud workflow scheduling such as the type of cloud environment, the type of cloud workflow scheduling algorithms and the constraints and objectives of cloud workflow scheduling. Finally, several research challenges in this area are proposed.