A support base for a child safety seat includes a shell body affixed with an anchor structure, an arm extending transversally relative to the shell body and having a rod pivotally disposed through a tube portion, the tube portion having an extension connected with a side segment of the anchor structure, two latch assemblies respectively connected with a left and a right end of the rod for fixedly attaching the shell body with an anchorage fixture of a vehicle, and a cushion structure for cushioning a displacement of the shell body and the anchor structure relative to the arm and the latch assemblies when car collision occurs. The cushion structure is arranged adjacent to a joint region where the side segment and the extension are connected with each other, or in the base between the tube portion and an inner sidewall of the shell body.