The non-circular cigarette is a new type of cigarette specification, and the shape of the cigarette is square. In order to characterize its mainstream smoke emissions and combustion performance characteristics, the mainstream smoke indicators, smoke hazard evaluation index (H index), and in vitro combustion performance indicators under ISO puff mode were analyzed, and the mouth-by-mouth release law of the conventional indicators of mainstream flue gas was investigated. The results showed that the volume was reduced by 21.3%, and the puff resistance of cigarettes increased by 20–50 Pa after molding. Under ISO smoking mode, the unit CO emission and an H index of mainstream smoke in the routine indexes of non-circular cigarettes were the lowest. As a whole, the mouth-by-mouth release of mainstream smoke conventional indicators of non-circular cigarettes showed a downward trend with the increase in the number of puffs.