  In this paper, we present a numerical method to study the predicted
lightcurves as a function of viewing angle. We extrapolate the fitting formulae
for the mass and velocity of tidal dynamical ejecta across a wide range of mass
ratio validated with 66 simulations and use them in the calculations of the
kilonova lightcurves. The calculated peak luminosity of a BH-NS merger kilonova
is typically about a few times $10^{41}\ {\rm erg\ s^{-1}}$, which is always
$\lesssim4.5\times10^{41}\ {\rm erg\ s^{-1}}$. This corresponds to the AB
absolute magnitudes fainter than $\sim -15\ {\rm mag}$ in optical and $\sim
-16\ {\rm mag}$ in infrared. Since the projected photosphere area of the
dynamical ejecta is much larger than that of the disk wind outflows, the
dynamical ejecta usually contribute to the majority of the kilonova emission
from BH-NS mergers. The fitted blackbody temperature and the shape of the
observed multi-band lightcurves are insensitive to the line of sight. The peak
time of the observed multi-band lightcurves, affected by the light propagation
effect, is related to the relative motion direction between the dynamical
ejecta and the observer. The observed luminosity varies with the projected
photosphere area determined by the viewing angles. However, the predicted peak
luminosity only varies by a factor of $\sim (2 - 3)$ (or by $\sim1\ {\rm mag}$)
for different viewing angles. When the short-duration gamma-ray burst afterglow
is taken into account, for an on-axis geometry, the kilonova emission is
usually outshone by the afterglow emission and can be only observed in the
redder bands, especially in the $K$-band at late times. Compared with
GW170817/AT2017gfo, the BH-NS merger kilonovae are optically dim but possibly
infrared bright. At the same epoch after the merger, the blackbody fitting
temperature of the BH-NS merger kilonovae is lower than that of
GW170817/AT2017gfo.
