In this study, Brassica napus L. hairy roots were used to construct an Agrobacterium rhizogenes-mediated heterologous expression system for the sweet protein Mabinlin Ⅱ (M12) from the endangered plant Capparis masaikai Levl.The M12-encoding gene was ligated into the plasmid pCAMBIA to obtain the recombinant vector pCAMBIA-M12, which was then transferred into B. napus L. hairy roots via Agrobacterium-mediated transformation. The successful construction of the expression system was confirmed by β-glucuronidase (GUS) histochemical staining and polymerase chain reaction (PCR). The transformed hairy roots were cultured in liquid medium at 25 ℃ and 150 r/min for one week. The culture was centrifuged, and the precipitate was collected and disrupted. The supernatant was collected as crude extract of M12. Doubleblind sensory evaluation and an electronic tongue were used to assess the properties of the crude extract, with a maximum sweetness intensity of 4.89. This study provides a new method for the efficient synthesis of sweet proteins in plant cells and offers a novel approach for the sustainable development of endangered plant resources.