Network based on distributed caching of content is a new architecture to alleviate the ongoing explosive demands for rate of multi-media traffic. In caching networks, coded caching is a recently proposed technique that achieves significant performance gains compared to uncoded caching schemes. In this paper, we derive a lower bound on the average rate with a memory constraint for a family of caching allocation placement and a family of XOR cooperative delivery. The lower bound inspires us how placement and delivery affect the rate memory tradeoff. Based on the clues, we design a new placement and two new delivery algorithms. On one hand, the new placement scheme can allocate the cache more flexibly compared to grouping scheme. On the other hand, the new delivery can exploit more cooperative opportunities compared to the known schemes. The simulations validate our idea.