The invention provides a circuit board heat radiator of a downhole water distributor. The circuit board heat radiator comprises N-electrode semiconductor couples, P-electrode semiconductor couples, metal guide bars, leads and heat radiation fins. When the heat radiator is mounted in the water distributor, the planar heat radiation fins are close to the circuit board, and the curved heat radiation fins are close to the inner wall of the water distributor; during working, the semiconductor materials in the heat radiator are connected by direct current, the current flows in from the N-electrode semiconductor couples and out from the P-electrode semiconductor couples, and when two different materials form a loop according to Peltier effect, one end of the loop absorbs heat, and the other end radiates heat. One side close to the circuit board is a cold end absorbing heat, the end close to the inner wall of the water distributor is a heat end radiating heat. The circuit board heat radiator solves the problem that a common heat radiator is hard to mount in narrow space of the downhole water distributor, and is simple in structure and novel in theory; the circuit board heat radiator can be designed optionally according to actual conditions and the problem of difficulty in heat radiation of downhole circuit boards is effectively overcome.