The invention relates to a polyethyleneimine and polyphosphoric acid polyelectrolyte complex, and a preparation method thereof, and belongs to the technical field of synthesis material. The preparation method comprises following steps: one part of ammonium polyphosphate and 40 to 200 parts of a solvent are introduced into a reaction container, the temperature in the reaction container is controlled to be 50 to 100 DEG C, and thermal insulation stirring is carried out for 10min to 5h; 1 to 4 parts of polyethyleneimine and 20 to 40 parts of a solvent are introduced into a glass container, and stirring is carried out for 10min to 1h; when an obtained ammonium polyphosphate solution is transparent, an obtained polyethyleneimine solution is added drop by drop in 10min to 1h, reaction is carriedout for 10min to 3h, an obtained product is cooled to room temperature, pumping filtration or centrifugation is adopted for separation of the product, washing with distilled water and ethanol is carried out for a plurality of times, and drying, grinding, and sieving are carried out so as to obtain the polyethyleneimine and polyphosphoric acid polyelectrolyte complex. The reaction conditions are mild and stable; the repeatability and yield are high; the preparation method is suitable for enlarged production; and the polyion complex is capable of improving a plurality of performances of polymers such as flame resistance and high temperature resistance.