Using 3,5-dimethyl-4-aminopyrazole as primary substance, 1,4-diamino-3,6-dinitropyrazolo[4,3-c]pyrazole (LLM-119) was synthesized by six steps of diazotisation, intramolecular cyclic condensation, nitration, oxidation, decarboxylation nitration and N-amination. The structures of intermediates and target compound were confirmed by means of IR, 1H NMR, ¹³C NMR and elemental analysis. The post-treatment method of 4-diazonium-3,5-dimethylpyrazole salt was improved, the yield was increased from 77.0% to 86.1% because of using the freezing crystallization method instead of extractive method, which could avoid the use of organic reagent and shorten operation process. The mechanism of N-amination reaction was discussed in order to analyze the reason of low-yield theoretically. Using differential scanning calorimetry (DSC) and thermogravimetric (TG) as thermal analysis methods, the thermal performance of LLM-119 was studied with thermal decomposition peak temperature of 232.05 ℃ and the results showed that LLM-119 had preferable thermal stability. The structure of LLM-119 was also estimated by a B3LYP method on 6-31G(d,p) basis set in order that its steady geometric configuration and bond order were obtained. The vibrational frequencies, IR spectra and thermodynamics properties in different temperatures of the system were obtained on the basis of vibrational analysis, the relations of the effects of temperatures on thermodynamics properties were also obtained. The density of LLM-119 calculated through the Monte-Carlo method was 1.85 g/cm³, which approached with the experiment value, and the predicted detonation velocity was 8733.94 m/s by the formula of Kamlet.