The invention discloses a method for extracting a Beta-dextran in highland barley bran and preparing a highland barley dietary fiber, belonging to the field of food processing. The method comprises the following steps of: extracting and centrifuging the highland barley bran taken as raw material at an alkaline condition by zymohydrolysis of secondary cellulose, thus obtaining a deposition A and asupernatant; depositing and centrifuging the supernatant under an acidic condition to remove albumen; adding a high-temperature resistant Alpha-amylase into the supernatant under a neutral condition for zymohydrolysis; centrifuging, depositing in water, depositing and re-dissolving the supernatant by water, centrifuging and taking the supernatant; repeatedly depositing the supernatant by a ammonium sulphate and obtaining the Beta-dextran product; adding a composite enzyme in the deposition A for zymohydrolysis; adding a hydrogen peroxide in the deposition A for decoloring; washing the deposition A repeatedly by water; obtaining a solid by centrifuging; and drying the solid at low temperature to obtain the highland barley dietary fiber. The method can sufficiently utilizes the highland barley bran, obtains two products of Beta-dextran and dietary fiber, effectively improves the economical additional value of the highland barley bran and has important realistic significance and wide application prospect for developing high-tech industries.