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Circinus X-1 (Cir X-1) is a neutron star binary with an elliptical orbit of16.6~days. The source is unique for its extreme youth, providing a key tounderstanding early binary evolution. However, its X-ray variability is toocomplex to reach a clear interpretation. We conducted the first high cadence(every 4 hours on average) observations covering one entire orbit using theNICER X-ray telescope. The X-ray flux behavior can be divided into stable, dip,and flaring phases. The X-ray spectra in all phases can be described by acommon model consisting of a partially covered disk black body emission and theline features from a highly-ionized photo-ionized plasma. The spectral changeover the orbit is attributable to rapid changes of the partial covering mediumin the line-of-sight and gradual changes of the disk black body emission.Emission lines of the H-like and He-like Mg, Si, S, and Fe are detected, mostprominently in the dip phase. The Fe emission lines change to absorption in thecourse of the transition from the dip phase to the flaring phase. The estimatedionization degree indicates no significant changes, suggesting that thephoto-ionized plasma is stable over the orbit. We propose a simple model inwhich the disk black body emission is partially blocked by such a local mediumin the line-of-sight that has spatial structures depending on the azimuth ofthe accretion disk. Emission lines upon the continuum emission are from thephoto-ionized plasma located outside of the blocking material.

X-ray spectral variations of Circinus X-1 observed with NICER throughout an entire orbital cycle