To raise output power of the sevencore photonic crystal fiber laser, the effective mode area of sevencore photonic crystal fiber must be increased, and strong coupling among cores is needed to ensure the same phase output. In order to analyse the relationship between effective mode area and coupling strength among cores more intuitively, according to characteristics of the intensity distribution of the sevencore photonic crystal fiber supermode, a calculation method of coupling length of sevencore photonic crystal fiber is giren. The effect of structure of sevencore photonic crystal fiber on the effective mode area and coupling length is analyzed with multipolar method and finite difference beam propagation method. Through continuous optimization of the structural parameters, a sevencore photonic crystal fiber with large outer layer air holes and small inner layer air holes is designed. Its effective mode area is 3703 μm2 and coupling length is 13310 μm.