The present invention relates to the technical field of digital communications, and in particular, to an LDPC decoder based on random bitstream update. The decoder comprises a decoding control unit, an input buffer unit, an information conversion unit, a variable node update unit, a check node update unit, a code word check unit, and a serial output unit. The LDPC decoder is based on the random calculation principle, a variable node unit structure and a check node unit structure are simple, and then the wiring difficulties are reduced. The variable node update structure is used for signal sharing, and the resources occupied by the unit are reduced. Different construction modes are adopted for variable nodes of different degrees, so that the resources of the decoder are less, and the decoding speed is faster. By adopting the decoding optimization strategy of "alternate parameters" and "re-decoding", the decoding performance can be improved. A connection line between two layers of nodes adopts a "half-flooding" technology, and the operation clock frequency and data throughput of the LDPC decoder are effectively improved. By adopting a ''double check'' technology, the decoding convergence speed is faster.