The invention discloses grass unbleached pulp and a preparation method thereof. The fracture length of the unbleached pulp is between 3.48 and 7.2km, the folding resistance is between 15 and 70 times, the tearability is between 180 and 275 mN, the whiteness is 33 to 48 percent ISO, and the beating degree is between 33 and 38 DEG SR. The unbleached pulp can be taken as pulp of corrugated paper, napkin paper and packing paper. The preparation method comprises the steps of cooking, oxygen delignification and so on, and the unbleached pulp is obtained by obtaining high-hardness pulp the hardness of which has a potassium permanganate value of between 16 and 28 and is equivalent to a Kappa number of between 24 and 50 through the cooking, and then performing oxygen delignification treatment by introducing oxygen and adding sodium hydroxide, or being matched with a pulp squeezing step and / or a disintegration step before the oxygen delignification. The use of the method can not cause the unbleached pulp to be polluted by a bleaching agent, and the strength of the pulp is protected at the same time, thereby preventing cellulose in the pulp from being degraded due to the influence of bleaching.