Isonicotinic acid and 1,3-dimethyl barbituric acid were applied as the indication reagents for the determination of HCN content in mainstream smoke using constant flow analyzer (CFA). The recovery rate of HCN was above 99% and the relation coefficient was 0.9996 in the linear range 6.2×10-2-15 (mg·L-1). The influence of buffer solution and reaction conditions on this method was also studied. Possible interferential effects caused by 14 kinds of anions and 15 kinds of metallic ions were surveyed to prove the stability of this determination method. Compared with pyridine-pyrazolone indication system, isonicotinic acid / 1,3-dimethyl barbituric acid indication system provided easier operation, better stability, higher sensitivity and better repeatability, and was more environment friendly.