Cloud computing, emerging as a new scheme, makes adjustment for the utilization of available resources by using certain service mode. As one of the wide application services in cloud computing, cloud storage provides data storage and business access externally. The remote mode of service provision will inevitably come up against security problems and furthermore these problems will bring in crisis of confidence to the development of cloud computing services. That is bound to impede the wide application of cloud computing services due to the lack of necessary reliability. Analysis for the essence of these problems indicates that the introduction of an external trusted third-party audit (TPA) mechanism is important and indispensable for the protection of data security and the reliability of services. This paper mainly reviews and analyzes the problems occurring in current development of cloud storage. Besides, we also focus on analyzing the problems caused by introduction of the TPA in more details and exploring the application prospects and solutions for the TPA mechanism in cloud storage. Inspired by the research work, we propose the idea of a file-sharing system, which is built on top of our service delivery platform, with the TPA mechanism deployed, and discuss the assurance of reliability for the system.