Abstract: To investigate the effects of K/Na ratio in burn additives in cigarette paper on the pore formation during cigarette paper pyrolysis and its effect on the release of HCN in mainstream cigarette smoke, the fiber pyrolysis behavior of cigarette paper was characterized by a FS300 fiber pyrolyzer. The micropore structure in cigarette paper was further characterized by scanning electron microscope (SEM), and the relationship between the temperature distribution in cigarette burning cone and the release of HCN in mainstream cigarette smoke was investigated by modularized fine thermocouple. The results showed that: 1) The increase of K/Na ratio in burn additives promoted the pore formation by fiber pyrolysis, increased the amount of micropores and improved the diffusivity and permeability of cigarette paper in the vicinity of char line, which facilitated HCN diffusion and air dilution; the release of HCN in mainstream cigarette smoke was thus decreased during puffing. 2) The increase of the K/Na ratio in burn additives decreased the combustion temperature of cigarette paper and the release of HCN during smoking. As a result, the release of HCN in mainstream cigarette smoke was significantly reduced with the maximum reduction at 27.3%.