古代地理知識常由真實與傳說共同組成。大食國與古代中國曾有諸多貿易外交往來，但關於大食的紀錄裏，常出現一則果實如人頭的奇樹傳說。梳理中文典籍與以阿拉伯文為主的西亞文獻中的「人木」傳說，本文認為這則故事或為較早期文化交流中傳入中國的異域知識。中國的「人木」傳說約在唐代出現，分為「人首花」與「小兒樹」兩種類型，其後始終保持著穩定的故事型態。阿拉伯故事中，「人木」傳說作為一種故事母題常與其它古老神話傳說結合，型態上則主要有果實如人頭、女人兩種。它們最初都是關於遠方島嶼的異族傳說，不過流傳之中，中國與大食逐漸將奇木生長地指向對方國境。釐清異域傳說之來源與流傳過程，是考察古代域外知識構成及天下想像的基礎。同一傳說在不同文化的傳播結果，既說明了異域想像存在著一些時代性的共同特徵，亦可反映出不同時代、民族對遠方想像之差異。

Cultural exchange between ancient China and the Arab empire (Dashi) increased greatly in the Middle Age; however, a mythical tree story was often cited as a description of “Dashi” in Chinese literature. By comparing the “Renmu” legend in Arabic and Chinese literature, this article argues that it may appear in China as a result of earlier culture contact with the Western Asia. The legend of “Renmu” in China can be traced back to Tang Dynasty, and divided into two branches. One is its flower looks like a human head while the other is the fruit shapes like a child. These story patterns were still visible until the Qing Dynasty. In Arabic stories, a mythical tree named “Waqwaq” grows somewhere near China, it bears women or human heads rather than fruit. As a motif, the story is often combined with other ancient myth and folklore. The circulation of this fantasy in both areas showed several similarities. They were originally descriptions about distant islands, and then became a symbol of faraway place so that Arabic and Chinese stories pointed the location of the mythical tree to each other gradually. To understand the composition of ancient foreign knowledge and exotic imagination, the origin of legends is the foundation. The propagation of legend in different cultures shows not only common characteristics in exotic imagination of the time but also the differences.