  Subdwarf A-type stars (sdAs) are objects that have hydrogen-rich spectra with
surface gravity similar to that of hot subdwarf stars but effective temperature
below the zero-age horizontal branch (ZAHB). They are considered to be
metal-poor main sequence (MS) stars or extremely low-mass white dwarfs (ELM
WDs). In this work, using the stellar evolution code, Modules for Experiments
in Stellar Astrophysics (MESA), we investigate the sdAs formed both by the
evolution of (pre-)ELM WDs in double degenerate systems (DDs) and metal-poor
main sequence with the single evolution models. We find that both of the
evolutionary tracks of ELM WDs and metal-poor MSs can explain the observation
properties of sdAs. However, the proportion between these two populations are
uncertain. In this work, we adopt the method of binary population synthesis of
both ELM WDs in the disk and metal-poor MS in the halo to obtain the
populations of ELM WDs and metal-poor MSs at different stellar population ages
and calculate their proportions. We find that the proportion of metal-poor MSs
to sdA population for a stellar population of 10 Gyr is $\sim$98.5 percent,
which is consistent with the conclusion that most sdAs ($>$95 percent) are
metal-poor MSs. And the proportion of ELM WDs (metal-poor MSs) to sdA
population increases (decreases) from 0.1\% (99.9\%) to 20\% (80\%) with the
stellar population ages from 5 to 13.7 Gyr.
