Aiming at the side inlet sudden expansion combustor of ramjet with double lower 90° inlet, a cavity (360° circumference) is added to form a combined flame holder to obtain the side inlet sudden expansion combustor (scheme 1); based on Scheme 1, a cavity with 360° circumference is changed into 270° circumference to obtain scheme 2. Based on the three-dimensional two-phase numerical calculation method, the simulation calculation was carried out under the condition of design point Ma4.5 and altitude 26 km, and the performance parameters of the combustor were given. At the same time, the Mach number, static temperature, static pressure and vortex structure distribution of the combustor were compared and analyzed. The results show that: the cavity flame holder can promote the circumferential movement of gas flow, improve the residence time of fuel, which is conducive to the mixing of gas flow and fuel; the cavity has little influence on the flow field upstream of the combustor inlet, but has great influence on the flow field downstream of the combustor and the cavity section; the combustion tends to center combustion, and there is a temperature gradient along the radial direction near the wall, so reasonable utilization is conducive to thermal protection.