This experimental study investigates the influence of geometric parameters, specifically the expansion ratio, on combustion pressure oscillations within a solid fuel ramjet (SFRJ). Experiments were conducted under controlled conditions with a total incoming flow temperature of 540K, total pressure of 0.78 MPa, and air mass flow rate of 0.3 kg/s. The main geometric configurations of the engine included the combustion chamber length, afterburner chamber length, inlet diameter, and fuel grain inner diameter, with the expansion ratio defined as the ratio of the fuel grain inner diameter to the inlet diameter. By analyzing the pressure data during stable operation, it can be observed that as the expansion ratio increases, the average pressure in the combustion chamber significantly decreases. Fourier transform analysis identified varying frequencies and amplitudes of pressure oscillations across different expansion ratios. The results indicate that the increase in expansion ratio has little effect on the first-order primary frequency. As the expansion ratio rises from 1.75 to 2.0 and ultimately to 2.57, the amplitude of pressure oscillation increases from 1.89 kPa to 2.0 kPa and then to 2.3 kPa. These findings validate the experimental design and provide important insights for optimizing the performance and stability of SFRJ.