A kind of automatic injector assembly, comprising: medicament reservoir, backstop are used to for drug being discharged from the medicament reservoir Driving element, it is formed by marmem, it has the first construction and the second construction, and it is configured to be increased to shape when shape changes temperature or more when the temperature of the driving element from first structural change to second construction, to drive the backstop of the injection tube mechanism to pass through the medicament reservoir And heating mechanism, it is configured to actively heat the driving element. Heating mechanism is the chemical heat element for being configured to generate heat by exothermic process, or includes: the fluid storage for keeping fluid Heater is configured to heat the fluid in the fluid storage Pump, is configured to for fluid being discharged from the fluid storage And connecting conduit, it is arranged to and the fluid is transported to the driving element from the fluid reservoir.
