The invention relates to the technical field of tunnel construction, in particular to a modular TBM cutter disc capable of performing variable diameter expanding excavation. The modular TBM cutter disc capable of performing the variable diameter expanding excavation comprises a quadrilateral structural centre cutter disc module which is provided with a centre rotary body, and fan-shaped structuralperipheral cutter disc modules which are arranged around the four sides of the centre cutter disc module; the four peripheral cutter disc modules are respectively positioned on the four sides of thecentre rotary body so that an annular cutter disc is formed; the centre cutter disc module is connected with each peripheral cutter disc module through two ejecting and pushing oil cylinders; the baseof each ejecting and pushing oil cylinder is fixedly mounted on the centre cutter disc module; a piston rod head of each ejecting and pushing oil cylinder is fixedly mounted on the corresponding peripheral cutter disc module; each ejecting and pushing oil cylinder is positioned on one side, close to a soil warehouse of a rounding cutter disc, so that the variable diameter expanding excavation canbe realized by pushing the peripheral cutter disc modules through the ejecting and pushing oil cylinders, and an excavation line is properly expanded so as to reserve a deformation amount; and therefore, the problem that large-deformation expanding excavation of soft rock with high crustal stress in tunnel construction is effectively solved.