Cyanogen (C₂N₂) is a new type of fumigant, a potential replacement of methyl bromide. However, there is still no standard method available at home and abroad for detecting C₂N₂ and its degradation product, HCN (hydrogen cyanide) in soil. A method for the determination of residues of C₂N₂ fumigant and HCN in soil using head-space capillary gas chromatography (HS-GC) with a nitrogen-phosphorus detector (NPD) was developed; and headspace parameters for pretreatment of C₂N₂ and HCN in the soil was optimized in this study; and quality control parameters such as precision of standard, limit of detection and limit of quantification for the method were validated in experiments. Results showed that incubated in solution of 30% phosphoric acid at 50℃ for 1 h, the sample with C₂N₂ and HCN concentration in the range of 0.1~10 mg kg^-1 displayed a linear relationship with correlation coefficients >0.99 between C₂N₂ and HCN concentration and chromatographic peak area. The use of this method to determine C₂N₂ and HCN in soil is subject to a RSD of 4.58% and 6.32% (n=5) , respectively．Of the method, the limit of detection (LOD) and the limit of quantification (LOQ) of C₂N₂ and HCN was 0.016 and 0.028 mg kg^-1 ( S/N=3), 0.053 and 0.093 mg kg^-1 ( S/N =10), respectively. This method was also proved to be convenient in operation, high in sensitivity and least in outside interference, and therefore, applicable to determination of C₂N₂ and HCN residues in soil.