We report experimental evidence of in-situ Mg77Cu12Zn5Y6 bulk metallic glass (BMG) matrix composite with extraordinary plastic strain of 18.5% and specific strength of 4.31×105 N·m·kg?1, which are the highest plasticity and specific strength in Mg base BMG alloys reported to date. The excellent mechanical properties are attributed to the formation of the composite structure which is composed of amorphous matrix and hexagonal-close-packed (hcp) Mg solid solution needle phase with a width less than 500 nm. As plastic phase, the Mg needles not only possess the capability of deformation but also have work hardening phenomenon effect. They give rise to multiple shear bands, and hinder the propagation of local shear band in amorphous matrix.