A child safety seat, comprising a seat body and a collapsible support device, which is installed on the child safety seat, wherein the collapsible support device comprises a first supporting part and a second supporting part, the second supporting part being pivotally connected to an end of the first supporting part to be capable of being collapsed to the first supporting part, another end of the first supporting part being pivotally connected to the child safety seat, the first supporting part being selectively rotated relative to the child safety seat to be located at one of a use position, where the first supporting part protrudes out the seat body, and a storage position, where the first supporting part is stored in the seat body, the second supporting part being selectively rotated relative to the first supporting part to be located at one of a supporting position and a collapsed position, wherein the collapsible support device further comprises another one of the first supporting part, the two first supporting parts being disposed at two opposite sides of the second supporting part, wherein, when the first supporting part is located at the use position, the child safety seat is at a first forward use state, wherein, when the first supporting part is located at the storage position, the child safety seat is at a rearward use state, and wherein the collapsible support device further comprises a limitation part and a rotation part, the rotation part rotatably passing through the second supporting part, the rotation part and the first supporting part being fixedly connected, an end of the limitation part being inserted into the first supporting part, another end of the limitation part extending into the second supporting part, the second supporting part having a rotation room for the limitation part to extend into, the second supporting part having at least one positioning protrusion, a first positioning area and a second positioning area being formed between the positioning protrusion and a sidewall of the rotation room, when the second supporting part is rotated to locate the limitation part in the first positioning area, the second supporting part being located at the supporting position, when the second supporting part is rotated to locate the limitation part in the second positioning area, the second supporting part being located at the collapsed position. wherein the collapsible support device further comprises another one of the first supporting part, the two first supporting parts being disposed at two opposite sides of the second supporting part, wherein, when the first supporting part is located at the use position, the child safety seat is at a first forward use state, wherein, when the first supporting part is located at the storage position, the child safety seat is at a rearward use state, and wherein the collapsible support device further comprises a limitation part and a rotation part, the rotation part rotatably passing through the second supporting part, the rotation part and the first supporting part being fixedly connected, an end of the limitation part being inserted into the first supporting part, another end of the limitation part extending into the second supporting part, the second supporting part having a rotation room for the limitation part to extend into, the second supporting part having at least one positioning protrusion, a first positioning area and a second positioning area being formed between the positioning protrusion and a sidewall of the rotation room, when the second supporting part is rotated to locate the limitation part in the first positioning area, the second supporting part being located at the supporting position, when the second supporting part is rotated to locate the limitation part in the second positioning area, the second supporting part being located at the collapsed position.