The present application discloses a softening device (200) by which an outer insulating layer of an elongate electrical heating element (121) to be secured onto a cushion, especially a vehicle seat or rest cushion is able to be softened, the softening device (200) comprising: a body (210), wherein the body is provided with a gas passage, a feeding passage (211), and an open cavity (213), wherein hot air is able to be driven through the gas passage, and the elongate electrical heating element (121) is able to be conveyed through the feeding passage (211), wherein the gas passage and the feeding passage (211) are isolated from each other but both of them open into the open cavity (213), wherein after passing through the feeding passage (211) and the open cavity (213), the electrical heating element (121) is able to be discharged out of the body (210). The present application also discloses an electrical heating element (121), a sewing apparatus comprising the softening device, and a method for controlling the softening device.