An electron density (ne) calibration method using a fiber-based interferometer (FI) for Thomson scattering (TS) diagnostics is proposed. An interferometer probe laser is injected into the plasma from the back face of a TS final bend mirror that has a transparency at the wavelength of the FI probe beam. The coaxial line electron density is measured by the FI, making it possible to calibrate neL from TS without either misalignment or optical loss in the collective optics of the TS system. In this paper, we report the conceptual design, wavelength selection of FI and potential issues in calibration of the fiber-based interferometer system. © 2012 IOP Publishing Ltd and SISSA.