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The study of multi-rate quasi-cyclic low-density parity-check (QC-LDPC) codes for small data transmission in space-based internet of things (IOT) scenarios, combined with the design method of LDPC code check matrix in 5G standard and the characteristics of fixed code length under different code rate conditions in digital video broadcasting’s second generation satellite television broadcasting standard extensions (DVB-S2X) standard, explores the design scheme applicable to the implementation of multi-rate QC-LDPC codes for short codes in satellite communications under fixed code length conditions, and conducts simulations to verify; the scheme is validated by simulation. The check matrix is designed by the algebraic method of QC-LDPC code superposition construction, combining shortening and expansion operations to construct a circular shift matrix with fixed coding length and multiple code rates, which maintains the Raptor-like structure of the 5G LDPC coding standard; finally, the designed scheme is compared with the IEEE802.11n standard for error According to the simulation results, the performance difference between the designed scheme and IEEE802.11n standard is small, and the bit error rate can reach 10-7, which achieves the expected result.