The invention provides a synthesis method of a large-area PbI<2> slice and establishment of a flexible photoelectric detector thereof. The synthesis method of the large-area PbI<2> slice is characterized by comprising the steps of adding lead chloride, iodine and thiourea into a reaction kettle, loading the reaction kettle with water which accounts for 80-90% of the total volume, performing stirring and ultrasonic processing, putting the product into a drying oven at a temperature of 120-150 DEG C to be maintained for 8-12h, and then performing natural cooling to the room temperature; and performing filtering, washing and drying on the product to obtain the PbI<2> slice. By virtue of the synthesis method, simple synthesis method for the solution, large product dimension, high yield, simpledevice establishment process, easy scale production, and high repeatability are achieved.