A low temperature stiffness measuring device and method of a diaphragm capsule assembly for a carrier rocket are provided. In the device, a lower bottom plate (2) is placed on a material testing machine base (1), and a lower fixed plate (15) is used to fix the lower bottom plate (2) through a bottom plate fixed bolt (16); the lower bottom plate (2) is provided with a support rod (3); the top of the support rod (3) is fixed with an upper bottom plate (4); the upper bottom plate (4) is fixed with a thermal isolation groove (5); inside the thermal isolation groove (5), a diaphragm capsule bracket (11) is used to fix the diaphragm capsule assembly; a diaphragm capsule (12) in the diaphragm capsule assembly is connected with a diaphragm capsule tension compression tooling (9); one filter neck of a diaphragm capsule outer shell in the diaphragm capsule assembly is blocked, the other filter neck is connected with one end of a hole plate (6), and the other end of the hole plate (6) is connected with a helium gas source; a switching-over tooling (7) is connected with a tension compression mechanism of a material testing machine; an insulating flange (8) is arranged between the diaphragm capsule tension compression tool (9) and the switching-over tooling (7); and a liquid discharging mouth (14) is installed at the bottom of the thermal isolation groove (5).