Laser solid forming (LSF) is a new type of advanced manufacturing technology, which can achieve the rapid fabrication of complicated metallic components with high performance. Multitudinous parameters that need to control in the LSF process make it difficult to accomplish the parameters optimization successfully. Once the unsuitable process parameters are set, it is very easy to produce corrugated surfaces during the LSF process. In order to realize the surface evenness control of thin-walled parts, the authors have established a mathematical analytical model to reflect the correlation relationship between the single cladding layer thickness and input process parameters. The novelty of this model is that a relationship between the single cladding layer thickness and input process parameters was first established. The results show that the single cladding layer thickness is mainly determined by the parameters, such as powder feed rate, defocusing amount, carrier gas velocity, laser beam spot, and scanning speed. Finally, the theoretical calculation results were verified via a series of experiments, and a practical application example of the model in the surface evenness control of thin-walled parts was given. The theoretical calculation results of the single cladding layer thickness correlate well with the experimental results.