The invention discloses a reactor for one-step preparation of dimethyl ether from biomass syngas, which comprises a fixed bed reactor, a plurality of reaction tubes are parallelly distributed in the reactor along the longitudinal direction; a reaction gas inlet is arranged at the top end of the fixed bed reactor, an exhaust outlet is arranged on a shell pass of the reactor which is near to the top end, overheated water outlets are arranged at the two sides of the reactor which are near to the exhaust outlet; a reaction gas outlet is arranged at the bottom end of the fixed-bed reactor, a liquid drainage port is arranged on the shell pass of the reactor which is near to the bottom end, and overheated water inlets are arranged on the two sides of the reactor which are near to the liquid drainage port. The reactor overcomes the shortcoming that the traditional fixed bed reactor is difficult to enlarge due to the thermal transfer limitation, makes full use of the advantages of high conversation efficiency, simple operation, good stability, low investment, low system maintenance cost, and the like of the fixed-bed reactor, designs a one-step dimethyl ether synthesis device with the larger scale and rapidly promotes the marketization of the technology of synthesizing liquid fuel from the biomass syngas.