  We propose a revision of the system developed by L'epine et al. (2007) for
spectroscopic M subdwarf classification. Based on an analysis of subdwarf
spectra and templates from Savcheva et al. (2014), we show thatthe CaH1 feature
originally proposed by Gizis (1997) is important in selecting reliable cool
subdwarf spectra. This index should be used in combination with the [TiO5,
CaH2+CaH3] relation provided by L\'epine et al. (2007) to avoid
misclassification results. In the new system, the dwarf-subdwarf separators are
first derived from a sample of more than 80,000 M dwarfs and a "labeled"
subdwarf subsample, these objects being all visually identified from their
optical spectra. Based on these two samples, we re-fit the initial [TiO5, CaH1]
relation, and propose a new [CaOH, CaH1] relation supplementing the [TiO5,
CaH1] relation to reduce the impact of uncertainty in flux calibration on
classification accuracy. In addition, we recalibrate the $\zeta_{TiO/CaH}$
parameter defined in L'epine et al. (2007) to enable its successful application
to LAMOST spectra. Using this new system, we select candidates from LAMOST Data
Release 4 and finally identify a set of 2791 new M subdwarf stars, covering the
spectral sequence from type M0 to M7. This sample contains a large number of
objects located at low Galactic latitudes, especially in the Galactic
anti-center direction, expanding beyond previously published halo- and thick
disk-dominated samples. Besides, we detect magnetic activity in 141 objects. We
present a catalog for this M subdwarf sample, including radial velocities,
spectral indices and errors, activity flags, with a compilation of external
data (photometric and GAIA DR2 astrometric parameters). The catalog is provided
on-line, and the spectra can be retrieved from the LAMOST Data Release web
portal.
