A flame retardant treatment method for light natural fiber material has the following process steps: the ultramicro fiber material fabric - bleaching or dyeing - inflaming retarding by low temperature soaking and expanding - baking for cross linking - oxidation, neutralization, water washing - product inspection, wherein, the condition of low temperature soaking is 15-30°C; 10-30 seconds, and expanding is carried out at 35-60°C; for 20-40 minutes. Baking for cross linking is carried out at 145-185°C; for 2-8 minutes. Then the ultramicro flame retardant fiber material is treated with purificant, which is nanoscale titanium dioxide (TiO 2 ) or/and zinc oxide (ZnO) and its ratio of weight is 1-12.0wt%, dispersant 0.2-1.5wt%; and the nano-composite dispersant is added in the water solution for the treatment.