In order to facilitate the reliability of data transmission of Spinal codes over the fading channel, performance analysis of Spinal codes is conducted, and an improved encoding structure is proposed. First, we derive an approximate frame error rate (FER) upper bound for Spinal codes over the Rayleigh fading channel in the finite block length (FBL) regime. Then, inspired by the FER analysis process, we propose an improved encoding structure, named self-concatenation structure, to reduce the FER of Spinal codes. In addition, a parallel structure is proposed for Spinal codes to improve the decoding throughput. For the self-concatenation structure, simulation results show that it exhibits a significant gain in anti-noise performance compared with the original Spinal codes over the Rayleigh fading channel. For the parallel structure, we find that by combining the parallel structure with the self-concatenation structure, not only is the encoding and decoding throughput of Spinal codes significantly improved but also the FER of Spinal codes is reduced.