The invention discloses a railway down-traversing highway tunnel double-layer casing pipe reinforcement double-pipe shed follow-pipe drilling construction method. The method comprises the following steps that construction preparation is made, wherein existing tunnel defects are surveyed; secondly, advance geology forecast is conducted; double-layer pipe shed advance pre-reinforcement is conducted, wherein the double-layer pipe shed advance pre-reinforcement is adopted, a follow-pipe drilling method is adopted in construction, each segment of a single-hole steel pipe is installed after being drilled by one segment, after each hole is formed, the hole can be grouted immediately, drilling gaps are filled, and a periphery surrounding rock is reinforced; fourthly, excavating and supporting are conducted, wherein excavating is conducted through a double-side-wall drift heading technology, a fracture full ring is excavated, and double layers of supports of a structural steel frame and a grid steel frame are arranged; fifthly, secondary lining is conducted, wherein a curved wall composite lining with an inverted arch is adopted; and sixthly, construction is finished, and monitoring and measuring are conducted. According to the construction method, the excavation technology is reasonably selected, the effective advance reinforcing measure is taken, and the tunnel down-traversing construction is stably and safely finished.