Abstract: This paper investigated the Cavern No.4 in Tang Dynasty Gold Cavern of Suichang Gold Mine. The surrounding rock of ancient passages is found to be hard metamorphic rock. Its Protodyakonov coefficient is as high as 14～19. From field research, four interesting phenomena are founded. Firstly, the joint planes in ancient passages have the feather of high density, high dip angel and small spacing. Secondly, the strike between axis in ancient passages and joint plane are basically consistent. Thirdly, the ratio of height to width in cross-section of ancient passages has the values ranging from 4 to 6,which is large. Fourthly, most of ancient passage axes are arc shape. As it is known, tensile strength and cohesion on joint planes are low. More than a thousand years ago, the ancients understood the above feather of joint planes with high dip angle and could make good use of them to excavate the passages. In order to take full advantage of the above feather, the ancients adjusted the direction of axis in ancient passages according to the strike of joint planes, and made them be consistent. Because the strikes of joint planes can change, the shape of ancient passages axis become arc. This shows that the ancients at the Tang Dynasty had mastered a considerable amount of geological knowledge, which was successfully applied in their engineering practice.