To improve the detection performance of the photoacoustic cell， a trapezoid compound photoacoustic cell is proposed and analyzed. Taking the traditional cylindrical resonant photoacoustic cell as the reference model， the reliability of the simulation method is verified by comparing the results of analysis and simulation. Based on the simulation method， the influence of the radius， length， and the number of the stepped cavity on the acoustic field and flow field characteristics of the trapezoidal composite photoacoustic cell is calculated. The results show that reducing the stepped cavity radius of the trapezoidal compound photoacoustic cell can enhance the photoacoustic signal， and the optimal length of the stepped cavity can make the photoacoustic signal to be the strongest， and the number of the stepped cavity should be one. In terms of the flow field， the configuration characteristics of the trapezoidal composite photoacoustic cell improve the condition of gas vortex return to the cavity. If the transition to the cavity is furthering rounded or chamfered， the velocity gradient in the cavity will become more stable. Select a set of design parameters， the calculation results show that the volume of the trapezoid compound photoacoustic cell cavity is reduced to 39.7% of the corresponding cylindrical resonant photoacoustic cell， and the photoacoustic signal is increased by about 18.7%. In addition， its frequency response bandwidth is narrowed， and its quality factor is improved. The overall results show that the acoustic field and flow field characteristics of the trapezoidal composite photoacoustic cell are better than that of the corresponding cylindrical resonant photoacoustic cell. The research content can provide reference for the structural optimization and improvement of the photoacoustic spectral photoacoustic cell.