The synthesis of 3,6-dinitropyrazolo[4,3-c]pyrazole(DNPP) was optimized at 100 g level with total yield increase from 9.3% to 17.9%. The structures of intermediates and DNPP were confirmed by IR, ¹H NMR, ¹³C NMR, elemental analysis and MS. The mechanism and safety of cyclization and reduction one pot method were discussed. Freezing crystallization technics, instead of extractive technics, which could avoid organic reagant and shorten operation process, was used to post-process 4-diazonium-3,5-dimethylpyrazole, increasing the yield from 77.0% to 86.1%. The single crystal of DNPP·H₂O was obtained in the water system. The crystal of DNPP·H₂O belongs to monoclinic system, space group P2(1)/c, cell parameters: a=0.3480(4) nm, b=1.4134(16) nm, c=0.9027(11) nm, α=90°, β =94.327(16)°, γ=90°, V=0.4427(9) nm³, Z=4, D_c=1.756 g·cm^-3, μ=0.164 mm^-1, F(000)=240, R₁=0.0503, wR₂=0.1391.