The invention discloses a method for preparing a paper base material through a needle broad-leaved mixed sulfate pulp total-chlorine-free bleaching chemical pulp. The method comprises the following steps of (1) using a needle broad-leaved mixed pulp obtained by bleaching through a biochemical method as a paper pulp raw material, dividing the paper pulp raw material into three parts, and respectively pulp-beating, wherein the pulp-beating degree of the first paper pulp ranges from 15 to 25 degrees SR, the pulp-beating degree of the second paper pulp ranges from 45 to 55 degrees SR, and the pulp-beating degree of the third paper pulp ranges from 60 to 75 degrees SR; (2) mixing the three paper pulps prepared in the step (1) according to a certain proportion so as to obtain a mixed pulp, and enabling the mass ratio of the first paper pulp, the second paper pulp and the third paper pulp in the mixed pulp to be 10 to 30:65 to 85:10 to 20; (3) mixing moulding or compression moulding the mixed pulp obtained the step (2) so as to obtain the paper base material. According to the method provided by the invention, a gradient pulp-beating method is adopted, and paper pulp fibers with different pulp-beating degrees are mixed, so that the paper base material can be ensured to have a favorable drainage property during a preparation process, and the prepared paper base material has higher strength.