This paper described the measuring principle of flexible joint′s angular stiffness. In order to do an experiment of flexible joint′s stiffness, a spiraltubet yped inductance displacement sensor and aforce sensor were developed. The force se nsor integrated a fourhole beam with a differential spiral tubetyped ind uctance displacement sensor. In addition, a measuring system of the flexible jo int ′s stiffness was constructed whose core were the both sensors mentioned abo ve. The re sult shows that the force sensor is effective and its relative error of repeated measurements is less than 1%．