The invention discloses a construction method for three-step six-step ultrashort step for a large-section tunnel. The method comprises the following step: 1) sequentially dividing the cross section of a tunnel into a first part, a second part, a third part and a fourth part from high to low, then excavating the first part, and sequentially constructing in-advance forepoling and preliminary bracing of the hole structure of the first part; 2) after the first part is excavated to N m, excavating the second part, prolonging a steel frame, constructing preliminary bracing and temporary inverted-arch bracing of the hole structure of the second part, excavating the third part and constructing preliminary bracing of the hole structure of the third part; 3) excavating the fourth part, constructing preliminary bracing of the hole structure of the fourth part, building the inverted arch of the hole within a preset period of time, and constructing two linings for the arch wall in the hole; 4) repeating the step 1), 2) and 3) to complete excavation and hole forming of the tunnel. The construction method effectively guarantees the stability of both the tunnel structure and the tunnel face, and is simple in procedure, high in speed and low in cost.