The reactions of FNCO to FOCN, FNCS to FSCN, and FNCSe to FSeCN have been studied at MP2/6311++G (2df)//B3LYP/6-311++G (2df) level. Geometries of the reactants, transition states, intermediates, and products have been optimized. The reason that FSCN is easily detected instead of FNCS has been explained. We predict that FNCO and FSeCN are more easily detected than FOCN and FNCSe, respectively. The breaking and formation of the chemical bonds in the reactions have been discussed by the topotogical analysis method of electronic density. The calculated results show that there are two kinds of structure transition states (the T-shaped conflict structure transition state and the bifurcation-type ring structure transition state) in the reactions studied.