Abstract:Helical magnetic flux rope (MFR) is a fundamental structure of corona mass ejections (CMEs) and has been discovered recently to exist as a sigmoidal channel structure prior to its eruption in the extreme ultraviolet (EUV) high temperature passbands of the Atmospheric Imaging Assembly (AIA). However, when and where the MFR is built up are still elusive. In this paper, we investigate a MFR in detail, whose eruptions produced a energetic solar flare and a CME on 2014 September 10. The AIA EUV images reveal that for a long time prior to their eruption, MFR is under formation, which is probably through magnetic reconnection between two groups of sheared arcades driven by the shearing and converging flows in the photosphere near the polarity inversion line. The Interface Region Imaging Spectrograph Si IV, C II, and Mg II lines exhibit a relatively large blueshift and an extremely strong non-thermal broadening at the formation site of the MFR. These spectral features consolidate the proposition that the reconnection plays an important role in the formation of the MFR. For the MFR, the reconnection probably takes place in the lower atmosphere and results in the strong blueshift and nonthermal broadening for the MgII, C II, and Si IV lines.