In recent years, low-temperature joining technology based on silver nanoparticles (AgNPs) pastes is one of the most promising electronic packaging interconnection technologies of power electronics. It has major applications in the field of energy electronics, wearable devices, and 3D printed circuits. AgNP paste has the advantages of low sintering temperature, high thermal and electrical conductivity, and reliable performance, which is particularly applicable to high-power applications. However, the essential mechanical properties of microporous structures in sintered AgNP, such as elastic modulus, ultimate tensile strength (UTS), and shear strength, have not been well studied. This paper clarifies the influence of different porosities on the shear strength and the fracture evolutions in the thermal interface materials. Phasefield modeling of sintered AgNPs with randomly distributed micro-pores was conducted with Abaqus and its subroutines. Random porous structures with different porosities, ranging from 10% to 35%, were generated. The porosity effects on fracture behaviors were analyzed with the mechanical properties and fracture processes obtained by phase-field modeling. Moreover, the formations and propagations of different crack patterns were illustrated in the complicated porous networks.