The present invention relates to a method and an auxiliary device in connection with disassembly and/or assembly of a tunnel thruster unit by use of the auxiliary device (1) to guide the thruster unit (2) and its movement during disassembly/assembly inside the tunnel (4), the thruster unit (2) thereafter being led away through the tunnel (4), said auxiliary device (1) being removably fixed at the thruster unit (2) before final disassembly, such that the auxiliary device (1) controls the movement during disassembly/assembly, mainly by compressive forces.