The invention relates to a chemical method for in situ synthesis of platy silver sulphide nano crystal photoelectric film at low temperature. The method includes that firstly sulphur powder is added into a container, and then organic solvent is added, volume of organic solvent is more than 1/2 of volume of the container, and finally base material with clean metal silver surface is placed at the bottom of the container in inclining or horizontal way, thus avoiding direct contact with sulphur powder. The base material and sulphur powder are immersed under solvent liquid level and react for 4-184 hours at the temperature of 20-60 DEG C, concentration of simple substance sulphur powder in reactant is maintained to be saturate, namely film material composed of platy silver sulphide nano crystal is prepared on the surface of the base material with clean metal silver surface by in situ, and product is washed by absolute ethyl alcohol and dried at room temperature. The method for preparing base material with clean metal silver surface includes that base material with metal silver surface is placed into absolute alcohol, is cleaned by an ultrasonic cleaner for 3min and then is soaked in DMF or absolute alcohol for spare use. The invention can be carried out at low temperature, has low energy consumption and is convenient and efficient, thus being beneficial to industrialized production.