Abstract: Taking hign density baler 9KG - 350 as testing apparatus, compression tests are performed for common Chinese wildrye, alfalfa and rolled wheat straw. Through testing analysis, a compression formula in different charge die and a mathmatics model for agricultural fibre materials are established, it is revealed that there are great difference between open-compression and sealed compression, at the same time, it is pointed out that there are two stages of loose and compression in the whole compression process. The test shows that the variaties of materials and charge die have great effect on the compression process, and that the limit charge (initial density) exists in the certain compression condition, and that different materials have different critical charge.