The present invention relates to a method for pre-treating biomasses, more particularly for pre-treating biomasses intended for a biorefinery or similar facility for producing biofuels. The method for pre-treating biomasses according to the invention comprises a shredding step (40) and, upstream of said shredding step, a soaking step (30), in particular by immersion in corresponding soaking vessels, and simultaneous removal of foreign bodies from said biomasses (30'). The present invention further relates to a plant for pre-treating biomasses (100) capable of implementing the above-described method and comprising to this aim a shredding station (140) for shredding the biomasses and, upstream of said shredding station, a soaking station (130), in particular comprising one or more soaking vessels (132).