Abstract: In the paper, we introduced the concept of open molding aiming at the shortage of closed biomass molding machine at room temperature and improved molding cavity on the basis of the hydraulic actuated equipment of closed molding machine. Through the mechanical analysis of biomass materials, we designed a variablediameter conical head with the open molding and verified our results using small wood shavings, sawdust and so on. The molding effects of biomass molding machine with variable diameters at room temperature were analyzed and the compression ratio range at high quality molding was determined.