1. A caching method for a multi-layer video network, wherein the multi-layer video network comprises at least one parent node and multiple child nodes connected to the parent node, comprising: collecting a user request arrival rate of each child node with respect to each content in a content set; and determining an optimal storage policy and an optimal upload bandwidth allocation policy of each child node with respect to each content based on the user request arrival rate, storage capacity and upload bandwidth of each child node, and a size of each content, wherein determining the optimal storage policy and the optimal upload bandwidth allocation policy of each child node with respect to each content based on the user request arrival rate, the storage capacity and the upload bandwidth of each child node, and the size of each content comprises: calculating, by each child node, a current storage policy of each child node with respect to each content by using the user request arrival rate of each child node, the storage capacity, the size of each content, and an iteration coefficient with respect to each content, and sending the current storage policy to the parent node; calculating, by the parent node, a current total content demand according to the current storage policy of the multiple child nodes, the user request arrival rate, and the size of each content, and transferring the current total content demand to each child node; calculating, by each child node, an upload bandwidth allocation policy according to the current total content demand and the upload bandwidth of each child node, and transferring to the parent node; and updating, by the parent node, the iteration coefficient according to the upload bandwidth allocation policy, the upload bandwidth, and the current total content demand, and returning to the step of determining the current storage policy until the current storage policy and the upload bandwidth allocation policy converge at the optimal storage policy and the optimal upload bandwidth allocation policy, wherein calculating, by each child node, the current storage policy of each child node with respect to each content by using the user request arrival rate of each child node, the storage capacity, the size of each content, and the iteration coefficient with respect to each content comprises: defining the current storage policy as follows in each child node: X ij k ⁡ ( t ) = { 1 , for ⁢ ⁢ k ∈ [ 1 , z ) ; 0 , for ⁢ ⁢ k ∈ [ z ,  K  ] , wherein X ij k (t) is the current storage policy, wherein z=min{h:Σ k=1 h s k >B ij }, wherein K is the content set, wherein k is each content in the content set K, wherein s k is a size of each content k, wherein B ij is the storage capacity, wherein i and j are non-zero positive integers, wherein h is a positive number, wherein when X ij k (t)=1, the child node stores the content k, wherein when X ij k (t)=0, the child node does not store the content k, and wherein t is the number of iterations of the iteration coefficient.