We systematically investigate the source confusion of massive black-hole binaries (MBHBs) for the Taiji space-based gravitational-wave mission. Source confusion arising from the overlap of signals in both time and frequency domains can degrade parameter recovery accuracy. To assess this effect, we simulate three representative MBHB population models to estimate the expected number of overlap events. Assuming 100 detections per year, only 0.31–4.2 overlaps are expected annually. Using the Fisher information matrix with the imrphenomd and imrphenomhm waveform models, we find that overlap significantly enlarges parameter uncertainties, while the inclusion of higher-order modes (HMs) effectively mitigates this effect. The cases most susceptible to severe confusion, characterized by $\mathrm{\Delta }{\mathcal{M}}_z/{\mathcal{M}}_z<0.2\%$, occur in fewer than 0.14% across the three population models. The full Bayesian analysis further corroborates the Fisher predictions and also reveals that HMs help break key parameter degeneracies, with or without signal overlap. These findings underscore the importance of incorporating HMs for accurate inference in future space-based observations.

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• Gravitational wave detection
• Gravitational waves
• Astrophysical & cosmological simulations
• Data analysis
• Numerical simulations in gravitation & astrophysics