The invention relates to a constant temperature water mixing valve which comprises a cold and hot water double-control faucet. The cold water inlet end and the hot water inlet end of the double-control faucet are connected with a valve body with a cold water channel and a hot water channel; a cold water inlet impeller is arranged in the cold water channel; a hot water inlet impeller is arranged in the hot water channel; the cold water inlet impeller and the hot water inlet impeller are coaxial; a balance channel is arranged between the cold water channel and the hot water channel; a hot water returning channel and a cold water returning channel are arranged in the middle of the balance channel; and a balance piston capable of simultaneously plugging the two returning channels is arranged in the balance channel. The constant temperature water mixing valve has the characteristics of being simple in structure, convenient to use, accurate in temperature controlling, safe and reliable.