The invention discloses a passive compliance impedance mechanism used on a mechanical arm of a service robot. The passive compliance impedance mechanism used on the mechanical arm of the service robot comprises a baffle, an eccentric cam, a bearing, a support, a rolling shaft, a first sliding block, a linear guide rail, a first spring guide rod on which a first spring is sheathed, a base, a base sliding block which is arranged on the base, a fixed tray, a second spring guide rod on which a second spring is sheathed and a base sliding groove, wherein the eccentric cam is rotatably installed on a support shaft of the support through the bearing; the rolling shaft is arranged on the first sliding block; the first sliding block is arranged on the linear guide rail; the baffle and the support are fixedly arranged on the base; one end of the first spring guide rod is fixedly arranged on the baffle and the other end of the first spring guide rod is sheathed in a through hole in the first sliding block; one end of the second spring guide rod is fixedly arranged on the side wall of the base sliding groove and the other end of the second spring guide rod is sheathed in a through hole in the base sliding block; a first limiting switch is arranged on the baffle; a second limiting switch is arranged in the base sliding groove. The passive compliance impedance mechanism used on the mechanical arm of the service robot has the advantages that when external impact is encountered, the external impact can be quickly relieved and the people can be prevented from being injured by the external impact.