The experiment conditions such as NaOH dosage, microwave radiation power, NaOH process time and bleaching processes assembling in multiprocessing bleaching of paper pulp were optimized by using the method of orthogonal test. The mechanism of straw pulp bleached by microwave radiation was studied by infrared spectroscopy (IR) and fibre analysis. The best conditions for multiprocessing bleaching of paper pulp could be as following that firstly, H,2,O,2, bleaching, microwave power was 240 W, process time was 2.5min, H,2,O,2, dosage was 5%, NaOH dosage was 4%; secondly, NaOH processing, microwave power was 200 W, process time was 1min, NaOH dosage was 3.5%; thirdly, NaClO bleaching, microwave power was 240 W, process time was 2.5 min, NaClO dosage was 4.2%, NaOH dosage was 1%. In optimal process, the whiteness of paper pulp could reach more than 78%ISO. IR results proved that the configuration of the lignin was almost the same as before reaction. The fiber analysis indicated that the form of the pulp fiber did not vary compared with that before reaction.