The invention provides a combined cooling low-pressure casting hub die provided with a constant-temperature and constant-pressure water tank. The die comprises a top die body, a bottom die body and a side die body. The side wall of the top of the top die body is close to the inner rim casting position. The side wall of the lower portion of the top die body is close to the spoke casting position. A constant-temperature and constant-pressure water cavity with the top open is formed in the side wall of the top of the top die body. A water tank cover plate is arranged on the constant-temperature and constant-pressure water cavity. A water inlet pipe and a water outlet pipe which communicate with the constant-temperature and constant-pressure water cavity are arranged on the water tank cover plate. A rim heat-insulation groove is reserved between the side wall of the top of the top die body and the side wall of the lower portion of the top die body. The top die body and the bottom die body are each provided with a body water-cooling water channel. The combined cooling low-pressure casting hub die has the beneficial effects that the die is low in bulk temperature and large in temperature gradient; compared with dies which are designed previously, setting time is shortened; rapid cooling can be achieved at the rim position; bad defects generated at the connection position between the wheel diameter and the root of each spoke due to that fact that setting time is too long are reduced; and product performance is improved.