  Gravitational lensing of gravitational waves (GWs) is a powerful probe of the
matter distribution in the universe. Here we study the lensing effect induced
by dark matter (DM) halos on the GW signals from merging massive black holes,
and we revisit the possibility of detection using the Laser Interferometer
Space Antenna (LISA). In particular, we include the halos in the low-mass range
of $10^5-10^9\, M_\odot$ since they are the most numerous according to the cold
DM model. In addition, we employ the matched-filtering technique to search for
weak diffraction signatures when the MBHBs have large impact parameters
($y\sim10^2$). We find that about $(20-40)\%$ of the MBHB in the mass range of
$10^5-10^6M_\odot$ and the redshift range of $4-10$ should show detectable
wave-optics effects. The uncertainty comes mainly from the mass function of DM
halos. Not detecting any signal during the LISA mission would imply that DM
halos are significantly more massive than $10^8\,M_\odot$.
