The invention relates to a flame retardant, a preparation method and purposes thereof. The flame retardant is called O-alkyl acrylamide dialkyl thiophosphate. The preparation method is that: 1mol new distilled N-hydroxyalkyl acrylamide is dissolved in 500ml dry aether and poured into a 1000ml four-neck reaction flask with a constant pressure funnel, a refluxing condensing tube and a magnetic stirrer, and 1.05mol distilled triethylamine is added, and the reaction temperature is kept at 0-50 DEG C, then 1mol distilled sulfur dialkyl phosphoryl chloride is dripped into the reaction flask through the constant pressure funnel and the time of dripping is controlled for 1-6h; reaction is kept for 10-48 hours under room temperature, then the reaction is stopped and filtration is made to take out triethylamine hydrochloride, the filtrate is washed with 5 percent sodium hydroxide solution, then is washed with distilled water, and distillation is carried out to remove the aether and water, finally drying is carried out with anhydrous magnesium sulfate so as to obtain the O-alkyl acrylamide dialkyl thiophosphate. The flame retardant of the invention is particularly applicable to be used as a comonomer to prepare flame retardant Orlon.