Here, we present the first report on the design and manufacturing process of trousers excavated at Yanghai cemetery (42°48′–42°49′N, 89°39′–89°40′E) near the Turfan oasis, western China. In tombs M21 and M157 fragments of woollen trousers were discovered which have been radiocarbon dated to the time interval between the 13th and the 10th century BC. Their age corresponds to the spread of mobile pastoralism in eastern Central Asia and predates the widely known Scythian finds. Using methods of fashion design, the cut of both trousers was studied in detail. The trousers were made of three independently woven pieces of fabric, one nearly rectangular for each side spanning the whole length from waistband to hemline at the ankle and one stepped cross-shaped crotch-piece which bridged the gap between the two side-pieces. The tailoring process did not involve cutting the cloth: instead the parts were shaped on the loom, and they were shaped in the correct size to fit a specific person. The yarns of the three fabrics and threads for final sewing match in color and quality, which implies that the weaver and the tailor was the same person or that both cooperated in a highly coordinated way. The design of the trousers from Yanghai with straight-fitting legs and a wide crotch-piece seems to be a predecessor of modern riding trousers. Together with horse gear and weapons as grave goods in both tombs our results specify former assumptions that the invention of bifurcated lower body garments is related to the new epoch of horseback riding, mounted warfare and greater mobility. Trousers are essential part of the tool kit with which humans improve their physical qualities.