The invention discloses a tunnel swelling rock section arch changing and expanded excavating construction method. The method includes the emergency processing reinforced temporary arching process andthe subsequent arch changing and expanded excavating process. The sequential unit construction process with an existing adjacent arch as a unit section is adopted for the temporary arching process andthe arch changing and expanded excavating process. Temporary arching is started from the entrance section of a tunnel, the temporary arching construction of the first unit is completed first, and then the temporary arching of the rest of units is completed in sequence; and then, started from the entrance section of the tunnel, the arch changing and expanded excavating construction of the first unit is completed, and then the arch changing and expanded excavating construction of the rest of units is completed in sequence. The method is designed according to the characteristic of the high freeexpansion rate of tunnel swelling rock, the anti-deformation capacity of tunnel preliminary bracing and cover and arch combined supporting is improved, the detaching sequence is arranged reasonably during arch changing, the tunnel preliminary bracing stability in the sleeve and arch detaching process is ensured, the tunnel clearance is effectively controlled after tunnel swelling rock is reinforced, and it can be ensured that the thickness of a second lining meets the design requirement.