The invention discloses a railway T-shaped beam prefabricated formwork device which comprises lateral formwork devices, end formwork devices and a bottom formwork device. The lateral formwork devices are used for being installed on the two sides of a prefabricated T-shaped beam and limiting the side face shapes of the prefabricated T-shaped beam. Each lateral formwork device comprises a top component arranged in the vertical direction, a bottom component and a plurality of middle components connected detachably, both the top component and the bottom component are installed on a telescopic support, and the grounding end of the support is arranged on a rail. The end formwork devices are used for being installed at the two ends of the prefabricated T-shaped beam and limiting the end shapes of the prefabricated T-shaped beam. A groove is formed in the inner side face, attached to the prefabricated T-shaped beam, of each middle component. A demolding strip is arranged in each groove and made from a shape memory alloy material. The demolding strips are kept in a strip shape in the normal temperature state. When the temperature rises to a threshold value, parts of the demolding strips bulge to form a wave shape, so that a plurality of gaps are formed between the formwork device and the prefabricated T-shaped beam, and demolding is facilitated. The invention further provides a demolding method of the railway T-shaped beam prefabricated formwork device.