Layer structured MAB phases (M=transition metal, A=IIIA or IVA group element, B is boron) are promising ternary borides for high and ultrahigh-temperature applications. Herein, a new MAB phase Y5Si2B8 consisting of alternative stacking of YB4 and Y3Si2 slabs along the [001] direction is investigated. Density functional theory (DFT) calculations on the electronic structure and chemical bonding reveal that this new MAB phase has diverse chemical bonding and properties similar to MAX phases. The strong covalent bonds in the two-dimensional B network on (001) plane in the YB4 slab and between Si atoms on (002) plane in the Y3Si2-slab warrant the high stiffness (E-x=288 GPa) of Y5Si2B8 in the ab plane and the weak Y2-Si and Y1-B2 bonds that connecting the YB4 and Y3Si2 slabs underpin the low Young's modulus in [001] direction (E-z=200 GPa). The low shear deformation resistance is due to the presence of the metallic bond and the weak bond within the B-6 octahedral. The possible slip systems are {001}< 100> and {110}< 111> . Based on the low shear modulus (G = 104 GPa) and Pugh's ratio G/B, Y5Si2B8 is predicted as a damage tolerant MAB phase. Y5Si2B8 is also predicted electrically conductive and the conductivity is higher in directions parallel to ab plane. In addition, temperature-dependent phonon and electron heat capacity are predicted based on the electron and phonon density of states analysis.