The invention relates to a surface modified magnesium alloy thin walled pipe, and a backward extrusion die and a preparation method thereof. The thin walled pipe is formed by a Mg-Zn-Sr alloy pipe and an anodic oxidation surface coating; the anodic oxidation surface coating is formed by magnesium silicate and magnesium oxide; the backward extrusion die comprises an upper die part and a lower die part; the upper die part comprises an upper die plate and an extrusion rod; the lower die part comprises a lower die plate, an extrusion cylinder, an extrusion female die and a core rod; and the extrusion female die is arranged in the extrusion cylinder. The preparation method comprises the following steps of (1) preheating a Mg-Zn-Sr alloy ingot, and utilizing the backward extrusion die for backward extruding to obtain an alloy pipe; (2) drawing the alloy pipe to obtain an alloy ultra-fin thin walled pipe; (3) adopting the alloy ultra-fin thin walled pipe as an anode, and a lead pipe as a cathode, electrolyzing to obtain the surface modified Mg-Zn-Sr alloy ultra-fin thin walled pipe. According to the magnesium alloy thin walled pipe provided by the invention, the elasticity modulus is ranged from 41 to 50GPa, the tensile strength is ranged from 270 to 300MPa, the ductility is ranged from 9 to 14 percent, and the hardness is ranged from 55 to 60HV.