With the rise and world wide deployment of cloud utilities,the principle of the cloud download is proposed to provide high-quality file content distribution by using dedicated servers as cloud cache to guarantee the data availability and enhance the data transfer rate.As the system scales up to a large population,how to design appropriate storage capacity of cloud cache is a key challenge for cloud download.In this paper,primary elements impacting on storage capacity are explored through deliberating in large-scale commercial cloud download system,i.e.general user usage pattern and available period.And based on statistical analysis of real-world traces,we formulate storage capacity related to these two elements,which is the original contribution different from all previous works.This model gives guidance of potential system policy design.Finally,the effectivity of this model is demonstrated by simulation results compared with empirical data of practical system.