A supporting device includes a first and a second support, a fixed washer, and a threaded fastener. The first support has a threaded structure extending along a rotation axis, and a first engaging structure. The second support is pivotally connected to the first support relative to the rotation axis and has a through hole aligning with the threaded structure. The fixed washer includes a washer portion and a second engaging structure. The washer portion is disposed on the second support and aligns with the through hole. The second engaging structure passes through the through hole to engage with the first engaging structure. The threaded fastener presses against the washer portion and passes through the washer portion and the through hole to engage with the threaded structure. When the second support and the first support rotate relatively, the fixed washer and the first support remain fixed relative to the rotation axis.