The invention discloses a hot end assembly of a car exhaust system. The hot end assembly comprises a gas inlet pipe, a catalyst carrier assembly and a gas outlet pipe. The hot end assembly further comprises a hoop strapped at the joint of the catalyst carrier assembly and the gas outlet pipe, a support matched with the hoop, and a connecting part connecting the hoop with the support. The hoop is provided with a first assembly hole and a second assembly hole. The connecting part is provided with a first matching part and a second matching part, wherein the first matching part is provided with a first matching hole which is flush with the first assembly hole, and the second matching part is provided with a kidney-shaped hole. The support is provided with a mounting part matched with the kidney-shaped hole, and a second matching hole which is flush with the second assembly hole. The mounting part is provided with a protrusion which is formed through inward pressing, and a mounting hole which is located beside the protrusion and is flush with the kidney-shaped hole, wherein the protrusion is at least partially contained in the kidney-shaped hole. The protrusion can achieve the rotation prevention function. The hot end assembly further comprises a first screw which penetrates through the mounting hole and the kidney-shaped hole and fixes the support and the connecting part together.