Sanyi Township is situated at the south of Miaoli County. It neighbors Houli Township of Taichung County by the Daan River. At its northeast, it connects to Sinlong Village and Singlong Village of Tongluo Township with Sanjiao Mountain (567 meters). The Guandao Range is the natural boundary of Sanyi, Jhuolan and Dahu. At its west, it borders Yuanli, Cyongjiaokeng, Nanshihlin, Fusing, Tongsiao with the famous Huoyan Mountain. At the north lies the Jhangshu Village of Tongluo Township.