The invention discloses a method for preparing hexyl-didecyl 
phosphorus oxide. The method comprises the following steps: adding didecyl 
phosphorus oxide, 1-
hexene and di-tert-butyl 
peroxide in a PTFE(
Polytetrafluoroethylene) reaction kettle, adding a small amount of industrial mixed 
decene, stirring and dissolving, rising the temperature to be 130 to 140 DEG C, reacting for 12 to 36 hours, detecting and completely reacting under <31>P-NMR (
Nuclear Magnetic Resonance), adding 5 to 15 weight percent of diluted 
hydrochloric acid in a reaction solution for carrying out demulsifying and washing, extracting by using trichloromethane, washing an 
organic layer to be neutral by using water, evaporating to remove trichloromethane and unreacted olefin through a 
rotary evaporator, recrystalizing by using 
ethanol, filtering, and carrying out 
vacuum drying, thus obtaining white 
solid. The method for preparing hexyl-didecyl 
phosphorus oxide, disclosed by the invention, is simple in steps, is green and 
environmentally friendly and is good in 
atom economy, a product is high in purity, separation is easy, and an industrial prospect is obtained; the product is applied to industries of water qualitydetection and 
sewage treatment.