Influence of temperature and optical irradiation is investigated on the parameters of elementary cell and linear coefficients of thermal expansion of Ag3AsS3 proustite crystals. Object of the study - single-crystals of compounds - semiconductor Ag3AsS3. Purpose of the study - to determine an elementary cell parameters change features of Ag3AsS3crystals in the interval of temperatures of T= 120-300 К with presence of optical irradiation. Method of the study - X-rays metod. Temperature dependences of the elementary cell parameters and also linear coefficients of thermal expansion of crystals of proustite Ag3AsS3 are investigated in the temperature range of T=120 - 300 K both in the darken mode and with an intensive laser irradiation. It is discovered, that there is a change of the parameter Ac «0,003A as a height at cooling and reduction at heating at the temperature of T «150 K on condition of laser irradiation. The jump of values of parameter of elementary cell represents the change of character of processes of deserializing of cations of Ag+ in transition from the darken mode to the intensive laser irradiation, because the size of silver ions transition potential barrier between octahedral cavities change as a jump. KEY WORDS: CRYSTALLINE STRUCTURE, PHASE TRANSITION, PARAMETERS OF ELEMENTARY CELL, PROUSTITE.