  This paper is devoted to the primary spectro-polarimetric observation
performed at the New Vacuum Solar Telescope of China since 2017, and our aim is
to precisely evaluate the real polarimetric accuracy and sensitivity of this
polarimetry by using full Stokes spectro-polarimetric observations of the
photospheric line Fe I 532.4 nm. In the work, we briefly describe the salient
characteristic of the NVST as a polarimeter in technology and then characterize
its instrumental polarization based on the operation in 2017 and 2019. It is
verified that the calibration method making use of the instrumental
polarization calibration unit (ICU) is stable and credible. The calibration
accuracy can reach up to 3$\times 10^{-3}$ . Based on the scientific
observation of the NOAA 12645 on April 5th, 2017, we estimate that the residual
cross-talk from Stokes $I$ to Stokes $Q$, $U$ and $V$, after the instrumental
polarization calibration, is about 4$\times10^{-3}$ on average, which is
consistent with the calibration accuracy and close to the photon noise. The
polarimetric sensitivity (i.e., the detection limit) for polarized light is of
the order of $10^{-3}$ with an integration time over 20 seconds. Slow
modulation rate is indeed an issue for the present system. The present NVST
polarimeter is expected to be integrated with an high-order adaptive optics
system and a field scanner to realize 2D magnetic field vector measurements in
the following instrumentation update.
