A child safety seat includes an ISOFIX assembly and a base. The ISOFIX assembly includes a support foot, a connecting portion, and a sliding rail portion between the support foot and the connecting portion. The connecting portion connects to a car seat. The sliding rail portion includes a first sliding rail portion and a second sliding rail portion connected to the first sliding rail portion. A width of the second sliding rail portion is larger than a width of the first sliding rail portion. The base has a frame assembly which has a sleeve portion slidably sleeved on the sliding rail portion. The sleeve portion includes first and second sleeve portions slidably sleeved on corresponding first and second sliding rail portions. The base is operable to be adjusted telescopically with respect to the ISOFIX assembly through a sliding cooperation between the sliding rail portion and the sleeve portion.