The present invention relates to a manikin when used in a child restraint system (CRS) test or a static test in a vehicle. In the current market, manikins are used to represent children in the CRS tests or relevant vehicle static tests and it is particularly important that the manikins simulate real children’s weight and size. However, conventional manikins are unable to accurately simulate the weight and size of a child, leading to inaccurate test results the present invention provides a manikin for use in a child restraint system (CRS) test or static tests in a vehicle, comprising a head portion configured to simulate the head of a child; a torso portion connected to the head portion through neck steel joints; a pair of arms connected to the torso portion through shoulder steel joints, the arms being positionable to simulate a seated child; and a pair of legs connected to a pelvis portion through hip steel joints, the pair of legs being positionable to simulate the seated child; characterized in that a plurality of cavities formed inside the manikin; and an interchangeable weight ballast disposed inside the torso portion to adjust the weight of the manikin near the weight of a normal child.