The utility model provides a silage fermentation cylinder is used in laboratory, including a jar body, jar body is for cylindrical hollow container, and the top of jar body is provided with the lid removablely, and the shaft hole has been seted up at the middle part of lid, and the one end of piston axle is passed the shaft hole and is extended jar internally, and the epaxial fixed piston that isprovided with of jar internal piston, piston set up in the jar internally along sealed slip of jar direction of height of the body, set up the tank body cavity's of intercommunication piston downsideinlet port in the piston axle along its length direction, and the inlet port correspondence of the epaxial end of piston is provided with one -way discharge valve. Both having realized the discharge of air, and can also having realized the compaction state of ensiling sample, furthest simulates silage's ideal conditions, reaches the mesh of the standardized preparation of ensiling sample to thereby that the ensiling quality that meets when solving laboratory analysis survey silage sample is difficult to is standardized, mass of the sample is widely different etc. Leads to the test result not have the representative problem. The utility model belongs to fodder grass processing production field.