An operating method and a circuit for low density parity check (LDPC) decoders, in which original bit nodes are incorporated into check nodes for simultaneous operation. The bit node messages are generated according to the difference between the newly generated check messages and the previous check node messages. The bit node messages can be updated immediately, and the decoder throughput can be improved. The required memory of LDPC decoders can be effectively reduced, and the decoding speed can also be enhanced.