Abstract: Beam width is one of important technical indexes to measure the performance of parametric acoustic array (PAA). The PAA's beam width should be calculated correctly in designing to ensure the performance of whole system. Taking the PAA formed by a circular piston transducer as an example, the Westervelt theory and the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation are used to calculate the PAA's beam width respectively. The accuracy of the numerical calculation of KZK equation in analyzing beam-width and its adaptability to far and near fields is verified by experimental results. The shortcoming of the Westervelt's formula for beam width calculation is indicated. By simulation, the relationships between the PAA's beam width and the frequency, absorption and array length are analyzed, and the influence of PAA's length on its beam width is explained in the physical sense.