The invention discloses a post-irradiation fuel rod underwater diameter measuring system, which comprises a spent fuel pool. A diameter measuring unit, which faces a fuel component, is arranged in the spent fuel pool. The diameter measuring unit is connected to a signal collecting and processing system. The operation is convenient. Through the micro displacement change between contact sheets, the displacement change is converted into the coil mutual inductance coefficient change, finally the displacement change is converted into voltage, the conversion between non electrical quantity and electrical quantity is realized, the diameter of a peripheral fuel rod of a fuel assembly can be rapidly and instantly measured underwater on site; a contact type electric signal transmission and conversion mode is adopted, and the measurement precision and detection efficiency are largely improved.