  We present the orbital period variability and evolutionary status of the W
UMa-type binary system V864 Mon from accurately measured fundamental
parameters. New $BV$ photometric observations of this system were performed in
2019 January and 2022 January, and the first high-resolution spectroscopic
observations were carried out on three nights between 2019 January and March. A
total of 29 times of minimum light were collected to examine the behavior of
the orbital period. Our analysis of these timings indicates a continuous period
increase at a rate of $+$2.62$\times$10$^{-7}$ d yr$^{-1}$ over the past 20
years, which can be interpreted as a mass transfer from the less massive
primary to the secondary component with a rate of 1.22$\times$10$^{-7}$
M$_\odot$ yr$^{-1}$. We measured the radial velocities (RVs) for both
components, and determined the effective temperature and projected rotational
velocity of the more massive secondary star to be $T_{\rm eff,2}$ = 5450 $\pm$
94 K and $v_2 \sin i$ = 192 $\pm$ 40 km s$^{-1}$, respectively, from the
comparison of the observed spectrum at the primary minimum and the theoretical
models. The individual masses and radii of both components were determined from
a simultaneous analysis of the light and RV curves, which are $M_1$ = 0.34
$\pm$ 0.02 M$_\odot$, $R_1$ = 0.69 $\pm$ 0.01 R$_\odot$, and $M_2$ = 1.06 $\pm$
0.04 M$_\odot$, $R_2$ = 1.16 $\pm$ 0.02 R$_\odot$, respectively. Our results
indicate that V864 Mon is a W-subtype of W UMa stars with time-varying spot
activity. The positions in the mass-luminosity and mass-radius diagrams
indicate that the secondary star belongs to the main sequence region, while the
hotter primary is located beyond the terminal-age main sequence.
