The invention discloses a phosphorus-containing flame retardant and a preparation method thereof, a device comprises a main frame body, the top end surface of the main frame body is internally provided with a uniform-mixing cavity, first hollow chambers are symmetrically arranged in the inner walls of the left and right sides of the uniform-mixing cavity, the outer side wall of each first hollow chamber is connected with a sliding chamber, the sliding chamber is slidably fitted with a sliding seat, a first compression spring is cooperatively arranged between the top end surface of the slidingseat and the top wall of the sliding chamber in a top pressing manner, a first electromotor is fixed in the right end surface of the sliding seat on the left side, a first turning shaft is arranged onthe right end of the first electromotor in a power cooperation manner, the first turning shaft is rotatably cooperatively connected with the sliding seat on the right side, the outer circumference ofthe first turning shaft in the uniform-mixing cavity is provided with a plurality of sets of mixing blades, first gears are fixedly symmetrically arranged on the left and right sides of the first turning shaft in the first hollow chamber, and a first transfer cavity is disposed in the sliding seat at the lower sides of the first gears.