Unsteady propagations of premixed flames in a mesoscale divergent channel were investigated experimentally. The impact of variable cross-section area and the flame-wall coupling on the flame transition between different regimes and the onset of flame instability was examined. A spinning wave with frequency ranged from 20 Hz to 150 Hz was observed and the results showed that the spin frequency was proportional to the flame speed. The wall temperature was controlled using cooling water or hot nitrogen and a transition from spinning wave to stable flame was observed, which proved that the spin was a result of the thermal coupling effect. The stable flame and spin flame could coexist within a bifurcation region caused by the thermal history effect of the wall. The critical flow rate for the onset of spinning wave highly depends on the equivalence ratio and but not the heating power.