Several previous works have shown that (Lenaite) AgFeS2 has a chalcopyrite structure with I-42d space group, but several information about its magnetic and optoelectronic behavior remain still unknown. The study of these properties is the main objective of our work, which is based on several theoretical approaches. The determination of the most stable magnetic ordering and the study of structural and elastic properties and that of most magnetic and optoelectronic properties have been carried out with FP-(L)APW + lo method, which is implemented in WIEN2k code. Our results have shown that AgFeS2-Chalcopyrite is an antiferromagnetic material with the same magnetic ordering as CuFeS2. The structural results are in good agreement with the previous results. The study of elasticity has shown that this compound is mechanically stable in its chalcopyrite structure and the analysis of the directional dependence of Young's modulus has shown its large elastic anisotropy. The electronic properties have been studied taking into account the strong correlation between the electrons of 3d-Fe states and their spin-orbit coupling (SOC). The obtained results with these two corrections have shown that AgFeS2 is a semiconductor with an indirect band-gap. Neel temperature has been estimated by mean-field approximation (MFA), which is based on the estimation of the exchange coupling parameters (J(ij)). These latter were calculated by SPR-KKR method and T-N found value is relatively very high in comparison with that of CuFeS2. Electronic transitions and optical absorption have also been studied respectively by the determination of the dielectric function and the study of the absorption coefficient variations as a function of energy.