5-Aminotetrazole was synthesized via a novel route by diazotization and intramolecular cyclization conditioning the reaction system at basic,and aminoguanidine nitrateas as starting material. The resulting 5-aminotetrazol was characterized by IR,¹³C NMR,and Ms etc. The mechanism of the reaction was proposed. The key factors affecting the yield,such as acidity or basicity of reaction system,reaction temperature and time, were also investigated. The optimized procedures are as follows: for diazo-reaction: pH=2~3,T=20~30 ℃ and t=0.5 h; for intramolecular cyclization: pH=8~9，T=85~90 ℃ and t=3.5 h. The yield of 5-aminotetrazole could be up to 77.2%.