Extremely large-mass-ratio inspirals (XMRIs), consisting of a brown dwarf orbiting a supermassive black hole, emit long-lived and nearly monochromatic gravitational waves in the millihertz band and constitute a promising probe of strong-field gravity and black-hole properties. However, dedicated data-analysis pipelines for XMRI signals have not yet been established. In this work, we develop, for the first time, a hierarchical semicoherent search pipeline for XMRIs tailored to space-based gravitational wave detectors, with a particular focus on the TianQin mission. The pipeline combines a semicoherent multiharmonic $\mathcal{F}$ statistic with particle swarm optimization and incorporates a novel eccentricity estimation method based on the relative power distribution among harmonics. We validate the performance of the pipeline using simulated TianQin data for a Galactic Center XMRI composed of a brown dwarf and Sgr A*. For a three-month observation, the pipeline successfully recovers the signal and achieves high-precision parameter estimation, including fractional uncertainties of $2.0\times {10}^{-6}$ in the orbital frequency, $2.9\times {10}^{-4}$ in the eccentricity, $2.5\times {10}^{-5}$ in the black-hole mass, and $5.6\times {10}^{-4}$ in the black-hole spin. Our framework establishes a practical foundation for future XMRI searches with space-based detectors and highlights the potential of XMRIs as precision probes of stellar dynamics and strong-field gravity in the vicinity of supermassive black holes.

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• Gravitational wave detection
• Gravitational wave sources
• Gravitational waves
• Astronomical black holes
• Data analysis
• Gravitational wave detectors