  When gravitational waves (GWs) pass by a massive object on its way to Earth,
a strong gravitational lensing effect will happen. Thus, the GW signal will be
amplified, deflected, and delayed in time. Through analyzing the lensed GW
waveform, physical properties of the lens can be inferred. On the other hand,
neglecting lensing effects in the analysis of GW data may induce systematic
errors in the estimating of source parameters. As a space-borne GW detector,
TianQin will be launched in the 2030s. It is expected to detect dozens of
mergers of massive black hole binaries (MBHBs) as far as z=15 and thus will
have high probability to detect at least one lensed event during the mission
lifetime. In this article, we discuss the capability of TianQin to detect
lensed MBHB signals.Three lens models are considered in this work: the point
mass model, the singular isothermal sphere (SIS) model, and the
Navarro-Frenk-White (NFW) model. The sensitive frequency band for space-borne
GW detectors is around millihertz, and the corresponding GW wavelength could be
comparable to the lens gravitational length scale, which requires us to account
for wave diffraction effects. In calculating lensed waveforms, we adopt the
approximation of geometric optics at high frequencies to accelerate
computation, while precisely evaluating the diffraction integral at low
frequencies. Through a Fisher analysis, we analyze the accuracy to estimate the
lens parameters. We find that the accuracy can reach to the level of 10^-3 for
the mass of point mass and SIS lens and to the level of 10^-5 for the density
of the NFW lens. We also assess the impact on the accuracy of estimating the
source parameters and find that the improvement of the accuracy is dominated by
the increasing of signal-to-noise ratio.
