Abstract: A measurement and operation auxiliary device, for supporting a length measurement tool disposed on an object under test, especially when measuring dimensions of a large object under test with a large length measurement tool, includes a fastening element, a connection shaft, and a support unit. The fastening element is unfastenably coupled to the object under test. The connection shaft is disposed on the fastening element. The support unit has a receiving chamber and a through hole. The connection shaft is penetratingly disposed in the through hole to allow the support unit to be rotatably coupled to the fastening element. The receiving chamber receives the length measurement tool. With the measurement and operation auxiliary device, the bulky length measurement tool can be fixed to a location suitable for measuring the dimensions of the large object under test without another co-worker and in a convenient, labor-saving manner.