Abstract: In order to overcome the limitation of paraxial Gaussian beams model losing accuracy in off-axis beam fields, a nonparaxial multi-Gaussian beam model for a rectangular aperture is proposed. With the help of this method, beam fields generated by an ultrasonic linear phased array transducer are calculated and compared with the corresponding fields obtained by Rayleigh-Sommerfeld integral model, paraxial multi-Gaussian beam model and Fraunhoffer approximation respectively. It is shown that nonparaxial multi-Gaussian beam mode is not limited by paraxial approximation condition, and can efficiently and accurately predict the radiation beam field of a linear phased array transducer over a wide range of steering angles.