An organosilicon flame retardant containing phosphorus and nitrogen and a preparing method thereof are disclosed. The method includes (1) dissolving amino silicone oil, adopted as a raw material, intoan organic solvent, adding triethylamine the weight of which is 15-30% of the weight of the amino silicone oil, and stirring the mixture in an ice water bath for a certain period of time; (2) dissolving a chlorophosphate compound the weight of which is 50-80% of the weight of the amino silicone oil into an organic solvent, slowly adding dropwise the solution to the amino silicone oil solution ata constant temperature, reacting the obtained mixture for 4-8 h, then removing the ice water bath, and terminating the reaction when the temperature reaches room temperature; and (3) removing unreacted chlorophosphate through aftertreatment to finally obtain a colorless thick oil substance. The method is simple in process, low in energy consumption, environmentally friendly, safe, good in repeatability and prone to industrialization. The prepared flame retardant is good in stability, nontoxic, capable of improving processing performance of plastic, low in influence on mechanical performance ofsubstrates and high in practicability.