The invention discloses a preparation method of the 9, 10-dihydro-9-oxa-10-phosphahenanthrene-10-oxide and the derivatives DDP, DDP-EG and DOPO-BQ. The invention has three steps to synthesize the DOPO: (1) preparation of CDOP, the O-phenylphenol (OPP) reacts with the Phosphorus trichloride (PCl3) with ZnCl2 as the catalyst. (2) purification of CDOP, vacuum distillation is carried out. (3) DOPO with high purity and yield is gained through the hydrolysis and crystallization. The preparation materials comprises the O-phenylphenol, the Phosphorus trichloride, the toluene, the alcohol, the deionized water, the itaconic acid, the ethylene glycol, the p-benzoquinone and the acetone. The invention simplifies the process conditions, improves the product purity with good production stability and repeatability. The purity of the product reaches over 99.0 per cent, the content of the chloride is below 50ppm, the OPP is about 0.01 per cent, the color is about 10 (10 per cent solution of acetone), the zinc-ion is less than 10ppm. The preparation method of the derivatives DDP, DDP-EG, DOPO-BQ is simple, good in stability and high in yield.