In this paper an improved overlap integral method based on the beam propagation method(BPM) is introduced to calculate the coupling coefficient to arrayed waveguides in an arrayed waveguide grating(AWG).The results obtained by the present improved method are compared with those obtained by a simple overlap integral method.The present method is validated by the comparison.The effects of the coupling between arrayed waveguides on the coupling coefficient are also analyzed.The effects become stronger when the separation between arrayed waveguides becomes smaller.