  We need to resolve the individual stars for binary fraction determinations of
stellar systems. Therefore, it is not possible to obtain the binary fractions
for dense or distant stellar systems. % We proposed a method to determine the
binary fraction of a dense or distant stellar system. The method is to first
determine the binary fraction variation for any two adjacent regions and then
add up those binary fraction variations along the radial direction to obtain
the binary fraction for a stellar system. Binary fraction variation is derived
by using ten binary fraction-sensitive spectral absorption feature indices
(SAFIs) and the binary fraction variation calibrations in terms of these SAFIs.
Using this method, we first presented the binary fraction variations for
twenty-one Galactic globular clusters (GCs). By comparisons, we find that they
agree well with the binary fractions based on the main-sequence fiducial line
method by previous studies. This verifies that the above mentioned method is
feasible. Next, we presented the binary fraction variations of thirteen
Galactic GCs. We gave the relationships between binary fraction and various
parameters, and found that binary fraction is negatively correlated with NHB
and NRR, binary fraction of some studies is not strongly correlated with NBS,
and the number of GCs with large binary fraction is greater at extreme blue
horizontal branch population ratio. At last, if we want to obtain more accurate
binary fraction, we suggest that the spectroscopic and photometric observations
are conducted at an appropriate area interval for a stellar system.
