The invention relates to a method for preparing cellulosic fibers from straws. The method comprises the following steps: slitting straws into strips by using a slitting mill, soaking and bleaching with hydrogen peroxide, refrigerating with an ultralow temperature refrigerating machine, oscillating in an ultrasonic oscillator and grinding into powder; adding the powder into a reactor, soaking with warm water for 1-2 hours according to a solid-to-liquid mass ratio of 1:3 to 1:5, adding bio-enzymes for performing enzymolysis for 1-3 hours, dewatering the bleached precipitate with a press master, drying by using a roller dryer so as to obtain dried cellulose powder; and mixing the cellulose powder with hot water so as to obtain a cellulose stock solution, and adding the cellulose stock solution to an electrostatic spinning machine for spinning the cellulosic fibers. The method disclosed by the invention has the beneficial effects that the strip-shaped straws are refrigerated by adopting the ultralow temperature refrigerating machine and oscillated and ground by utilizing the ultrasonic oscillator, and the cellulose structure can be prevented from being damaged, so that the cellulosic fibers are difficult to break in the spinning process; and with the adoption of the bio-enzymes prepared by mixing multiple enzymes, xylogen in the straws can be completely degraded.