A portable bioreactor is provided for driving displacement of at least one stirring sleeve relative to at least one tube cassette in a first direction and includes a machine frame unit and a first elevator. The first elevator includes a first linear movement module, a first transmission module, and a first turning module. The first linear movement module includes a first slider which is slidable on the first guide rail in the first direction between a first top position and a first bottom position. The first turning module is coupled to the first slider through the first transmission module so as to permit turning of the first turning module to be translated by the first transmission module into linear sliding movement of the first slider at a varying speed.