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The present study investigates the effect of the exit geometry of the main stage on thermoacoustic instabilities in a model combustor with a dual-swirl layout. In practical combustors, the main stage has a much larger airflow than the pilot stage, therefore dominating the aerodynamics of the global flame. In this paper, six different geometries of the main-stage exit are designed to cover the common styles in engineering applications. Their effects on thermoacoustic instability and flame shape are examined experimentally. Sudden transitions from the attached flame to the blurred flame are found in most of the exit geometries, excluding the outer-rough case. This transition of flame shape triggers the onset of strong thermoacoustic oscillation, leading to a jump in the pressure fluctuation amplitude. Frequency drop and the acoustic mode shift are also found at the transition point. Flame dynamics are further analyzed using the dynamic mode decomposition method. It is concluded that the outer-rough case has the best thermoacoustic stability within the tested range. The mechanisms of the observed phenomena are analyzed by measuring the reacting flow fields. It is found that the increased roughness of the outer wall can prevent the flame shape transition and the onset of thermoacoustic instability, possibly due to the weakened vortex structures and strain rate in the outer shear layer. [ABSTRACT FROM AUTHOR]