Abstract: Mild detonating fuse (MDF) plays a vital role in linear explosive separation devices. Energy output efficiency of mild detonating fuse in cylinder tube has significances to the structure design of retainer of protecting. To estimate the energy output efficiency of mild detonating fuse, a deformation cylinder tube testing set for small counts of explosive charges was developed. Three kinds of aluminum alloy tubes with the same inner diameter (3.2 mm), the same length (100 mm) and different outer diameters (20.0, 21.9 and 24.0 mm) were used in the tests, which were loaded by a MDF with outer diameter of 2.94 mm. The particle velocities histories of outer surface and deformation results of recycled specimen were measured. Plastic strain energy of the deformation cylinder tube can be calculated based on the all-plastic assumption. The energy output efficiencies of the MDF in three aluminum alloy cylinder tubes are 62.3%, 61.3% and 60.9%, respectively.