The photoacoustic cell， as the core component of the photoacoustic spectroscopy gas detection system， directly affects the detection accuracy of the system. Based on the classic cylindrical photoacoustic cell， the finite element analysis software is used to combine the two physics of pressure acoustics and thermoviscous acoustics and build a model to simulate the acoustic-thermal coupling process in the photoacoustic cell. The effect of different geometric parameters of the resonant cavity and buffer cavity on the performance of the photoacoustic cell is compared through simulation， and then， the optimal size is determined. The simulation results show that the length and radius of the resonant cavity and the buffer cavity will affect the resonance frequency and acoustic pressure. After fully considering the comparison results and the difficulty of industrial manufacturing， the optimal length of the resonant cavity is selected as 120 mm， the optimal radius is 3 mm， and the buffer cavity radius is 35 mm. On this basis， a photoacoustic cell with a rounded connection between the resonant cavity and the buffer cavity is designed. Compared with the right-angle photoacoustic cell with the same size， the round-corner photoacoustic cell has more advantages in improving the photoacoustic signal and reducing the flow noise interference. The quality factor is increased to 1.109 times， the cell constant is increased to 3 635.1 Pa·cm/W， and the acoustic pressure is increased to 1.26×10^-5 Pa. In the concentration detection of methane gas， the system sensitivity can reach 0.87 ppm， and the detection result is ideal， which meets the requirements of high sensitivity. Therefore， the performance of the round-corner photoacoustic cell has been significantly improved， which can provide a reference for the optimal design of the photoacoustic cell.