Abstract: The reaction mechanism of HNCS (isothiocyanic acid) and C2H(X2Π) radical has been investigated by using the density functional theory (DFT) at the B3LYP/6-311+G** level. The single point energies along the minimum energy path have been further refined at the UQCISD (T)/6-311+G** level. The dual-level direct dynamics method was employed to gain the information of potential energy surface for the reaction of HNCS with C2H(X2Π) radical. The rate constants were calculated over the temperature range of 250～2500 K by using variational transition state theory, and the small-curvature tunneling is included. The results show that the reaction mechanism of HNCS and C2H(X2Π) radical involves three path and NCS+C2H2 are main products. It is found that the rate constants of the reaction are positively dependent on temperature. The variational effect for the rate constant calculation is very small in whole temperature range and the small-curvature tunnelling effect is only important in the lower temperature range. The reaction is exothermic.