Abstract: In this paper horizontal directivity of arc array is discussed detailly by Using stationary phase method and more accurate formulae of mainbeam width and average pressure within mainbeam are given.The methods of amplitude shading and unequally spaced array given in this paper can successfully used to resolve the problem concerning the more obvious fluctuation of directivity functions within the mainbeam,and at the same time keep the mainbeam width and average pressure within the mainbeam approximately constant.