The invention provides a porous large-section cast-in-situ box girder and a construction method. The method comprises the following steps that 1, foundation treatment is conducted, specifically, stepsare excavated on an original foundation, and mortar plastering construction is conducted by adopting a Z-shaped dragging sliding device for carrying out foundation construction; 2, a support is set,specifically, after the foundation reaches a certain strength, a support system is set up; 3, a bottom die is mounted, and specifically, the bottom die of the box girder is mounted; 4, the support ispre-pressed, and specifically, the support system is pre-pressed by using a water bag; 5, an inner die is mounted; 6, box girder construction is conducted; 7, a box girder formwork is removed; and 8,the support system is removed. Aimed at the porous large-section cast-in-situ box girder ultrahigh full framing system construction method, the engineering cost is effectively reduced, the construction speed is accelerated, and the good technical economic benefits can be achieved by applying the method to practical engineering.