As the complexity of the mechanical system is increasing, designers need to give comprehensive consideration to the novelty, excellent performance and manufacturing feasibility of the structure in the design of the theoretical model of modern mechanism. However, the traditional manufacturing methods impose great restriction on the design. Selective laser melting (SLM) is one of the technologies that have most development potential, which can achieve direct manufacturing of metal functional parts from any complex computer-aided design (CAD) theoretical models in theory. Based on the characteristics of the freeform manufacturing of SLM, combining with the related research of South China University of Technology, we study the freeform design and direct manufacturing process of complex metal pieces with non-assembly, functional integration and lightweight characteristics, which provides effective reference for the innovative design and personalized manufacturing of products in the fields of aerospace, medical treatment and automobile.