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Ge-Sb-Te compounds (GST), the well-known phase-change materials, are considered to be promising thermoelectric (TE) materials due to their decent thermoelectric performance. While Ge₂Sb₂Te₅ and GeSb₂Te₄ have been extensively studied, the TE performance of GeSb₄Te₇ has not been well explored. Reducing the excessive carrier concentration is crucial to improving TE performance for GeSb₄Te₇. In this work, we synthesize a series of Se-alloyed GeSb₄Te₇ compounds and systematically investigate their structures and transport properties. Raman analysis reveals that Se alloying introduces a new vibrational mode of GeSe₂, enhancing the interatomic interaction forces within the layers and leading to the reduction of carrier concentration. Additionally, Se alloying also increases the effective mass and thus improves the Seebeck coefficient of GeSb₄Te₇. The decrease in carrier concentration reduces the carrier thermal conductivity, depressing the total thermal conductivity. Finally, a maximum <i>zT</i> value of 0.77 and an average <i>zT</i> value of 0.48 (300–750 K) have been obtained in GeSb₄Te_5.5Se_1.5. This work investigates the Raman vibration modes and the TE performance in Se-alloyed GeSb₄Te₇ sheddinglight on the performance optimization of other GST materials.