Abstract: The influence of the resonator shape on nonlinear acoustic field in a thermoacoustic engine is studied.The resonator of tiremoacoustic engine is boundary driving by a piston at one end,and the other end of it is rigid closed.A one-dimensional wave equation that accounts for gas dynamic nonlinearities and viscous dissipation in the resonator is established based on the governing equations of viscous hydromechanics.The nonlinear wave equation is solved using approximate Galerkin method.The nonlinear acoustic field in four different types of varying cross section resonators including hyperbolical,exponential,conical and sinusoidal are obtained and compared with that of a cylindrical resonator. It is found that the amplitude and waveform of pressure oscillations are strongly affected by the resonator shape,the driving amplitude and frequency of the piston.Waveform distortion,resonance frequency shift and hysteresis are observed,when the piston oscillation amplitude is large enough.The advantages of varying cross section resonator in thermoacoustic engine are revealed in inhibition of higher order harmonics,improvement of pressure ratio,etc.