  In this paper, we derive a 90\% credible lower bound on the modified
parameter of scalar-tensor theories as $\varphi_{-2}>-7.94\times10^{-4}$ by
using dominant-mode correction. Specific to BD theory, we have the constraint
$\omega_{\rm BD}>4.75$. Asymmetric binary systems usually have a significant
mass ratio; in such cases, higher harmonic modes cannot be neglected. Our work
considers higher harmonic corrections from scalar-tensor theories and provides
a tighter constraint of $\varphi_{-2}>-7.59\times10^{-4}$. Transitioning to the
BD theory, the constraint is $\omega_{\rm BD}>5.06$, with a 6.5\% improvement.
We also consider a plausible NSBH event, GW190814, which is a highly unequal
mass ratio source and exhibits strong evidence for higher-order multipoles. We
obtain poorly converged results when using the dominant mode while getting a
constraint of $\varphi_{-2}>-6.60\times10^{-4}$ on scalar-tensor theories when
including the higher harmonic modes. This suggests that the difference between
the dominant mode and higher modes has a significant impact on our analysis.
Furthermore, treating this suspected event as an NSBH event, we find
$\omega_{\rm BD}>6.12$ when including the higher harmonic modes. Combining
GW200115 and GW190814 and including higher modes, the constraint is improved to
$\omega_{\rm BD}>110.55$. This is currently the strongest constraint utilizing
GWs, contingent upon GW190814 being an NSBH event. Additionally, we take into
account a BD-like theory, known as screened modified gravity (SMG), and
investigate the coupling constant constraints, both with and without
higher-mode corrections, by using data from both GW200115 and GW190814.
