A method for preparing a straw pulp, comprising: putting a straw material into a cooker, adding a cooking liquor, and then heating the cooking liquor to 100 - 200°C, increasing pressure to 0.3 - 0.9MPa, keeping cooking for 150 - 250 minutes, and obtaining the straw pulp after pressing and washing; wherein in the cooking liquor, ammonium sulfite is used in an amount of 5 - 20% of bone dry raw material, sodium hydroxide is used in an amount of 0 - 15% of the bone dry raw material, and the liquor ratio is 1:2-15; wherein the method further comprises an oxygen delignification step after cooking or washing, the oxygen delignification step comprising: 1) regulating a concentration of the pulp which is obtained after cooking to 8-18%; 2) pumping the pulp to an oxygen delignification reaction tower and adding sodium hydroxide and oxygen; 3) the pulp being subjected to delignification reaction in the oxygen delignification reaction tower, wherein, the oxygen delignification is single stage and carried out in the oxygen delignification reaction tower; during the oxygen delignification, a temperature and a pressure of the pulp is respectively 95 - 100 °C and 0.9 - 1.2MPa at the inlet of the reaction tower, and 100 - 105 °C and 0.2 - 0.4MPa at the outlet; alkali used in the oxygen delignification is 2 - 4% of bone dry pulp based on sodium hydroxide, oxygen is added in an amount of 20 - 40kg per ton of bone dry pulp; and the pulp reacts in the reaction tower for 60 - 90 minutes. putting a straw material into a cooker, adding a cooking liquor, and then heating the cooking liquor to 100 - 200°C, increasing pressure to 0.3 - 0.9MPa, keeping cooking for 150 - 250 minutes, and obtaining the straw pulp after pressing and washing; wherein in the cooking liquor, ammonium sulfite is used in an amount of 5 - 20% of bone dry raw material, sodium hydroxide is used in an amount of 0 - 15% of the bone dry raw material, and the liquor ratio is 1:2-15; wherein the method further comprises an oxygen delignification step after cooking or washing, the oxygen delignification step comprising: 1) regulating a concentration of the pulp which is obtained after cooking to 8-18%; 2) pumping the pulp to an oxygen delignification reaction tower and adding sodium hydroxide and oxygen; 3) the pulp being subjected to delignification reaction in the oxygen delignification reaction tower, wherein, 1) regulating a concentration of the pulp which is obtained after cooking to 8-18%; 2) pumping the pulp to an oxygen delignification reaction tower and adding sodium hydroxide and oxygen; 3) the pulp being subjected to delignification reaction in the oxygen delignification reaction tower, wherein, the oxygen delignification is single stage and carried out in the oxygen delignification reaction tower; during the oxygen delignification, a temperature and a pressure of the pulp is respectively 95 - 100 °C and 0.9 - 1.2MPa at the inlet of the reaction tower, and 100 - 105 °C and 0.2 - 0.4MPa at the outlet; alkali used in the oxygen delignification is 2 - 4% of bone dry pulp based on sodium hydroxide, oxygen is added in an amount of 20 - 40kg per ton of bone dry pulp; and the pulp reacts in the reaction tower for 60 - 90 minutes.