A synthesis method which comprises reacting NH group-containing compounds with thiocyanates, cyanamides, nitrites or esters in the presence of a silylating agent to synthesize the corresponding nitrogen-containing addition or substitution products. This method not only enables the direct and efficient synthesis of nitrogen-containing compounds including isothioureas, guanidines, amidines and amides, but it also has a wide range of applications and is suitable for large-scale synthesis.