Li1+x FePO4 (0≤ x ≤3) as anode material for lithium ion batteries has been studied using ab initio calculations. Results show that large amount of lithium ions can be intercalated into LiFePO4 host. The structure changes continuously when the first two Moles of lithium ions (x ≤2) are intercalated into the LiFePO4 host, accompanied by large volume expansion (37.4% and 25.4% for the first and second Mole). The final product of Li3FePO4 possesses a stable chained structure, which is favorable for storing even more lithium. In the same time, lithium ion diffuses in a three-dimension pathway within the chained structure. The unit cell volume increases only by 4.9% from Li3FePO4 to Li4FePO4, and the chained structure keeps unchanged.