Density functional theory (DFT) calculations at the B3LYP/6-31G(d,p) theoretical level were performed for two novel explosives (compounds B and C) based on the guanidine-fused bicyclic skeleton $C_4N_6H_8$ (A). The heats of formation (HOFs) were calculated via isodesmic reaction. The detonation properties were evaluated by using the Kamlet-Jacobs equations. The bond dissociation energies (BDEs) for the thermolysis initiation bond were also analyzed to investigate the thermal stability. The results show that the compounds have high positive HOF values (B, 1064.68 $kJ{cdot}mol^{-1}$; C, 724.02 $kJ{cdot}mol^{-1}$), high detonation properties (${ ho}$, D and P values of 2.04 $g{cdot}cm^{-3}$ and 2.21 $g{cdot}cm^{-3}$, 9.98 $km{cdot}s^{-1}$ and 10.99 $km{cdot}s^{-1}$, 46.44 GPa and 59.91 Gpa, respectively) and meet the basic stability requirement. Additionally, feasible synthetic routes of the these high energy density compounds (HEDCs) were also proposed via retrosynthetic analysis.