Tobacco stem was delignified by means of the oxygen alkali technology and was then used to produce to- bacco sheets.The variations of harmful components in the cigarette smoke were investigated,and the optimal condi- tions of oxygen alkali technology were determined as follows: a liquid-solid ratio of alkali liquor to tobacco stem of 10mL/g,a mass ratio of potassium hydroxide to tobacco stem of 35%,a reaction temperature of 120℃,a reaction time of 40min,an oxygen pressure of 0. 6MPa and a dosage of protective agent MgCO₃ of 0. 5%.It is found that,as compared with the traditional water soaking pretreatment,the pretreatment with oxygen alkaline is more effective because it helps to reduce the contents of lignin,phenol,pyrocatechol and the total harmful substance by 47. 9%,61%,25% and 38%,respectively.