It is difficult to form aluminum alloy stablely by laser metal deposition(LMD) due to high thermal conductivity, low laser absorptivity, and significant heat accumulation effect. To realize automatic and accurate preheating, eliminating heat accumulation, analyzing the effect of preheating and improving the forming ability of AlSi10Mg aluminum alloy LMD, the technology of laser metal deposition with "internal powder feeding" and Ar supply protection was adopted, the laser preheating and fluid cooling temperature control system was designed and the preheating and cooling temperature control model was established, LMD forming experiments of AlSi10Mg aluminum alloy was carried out, the effects of preheating on laser absorptivity, surface quality, cross-section morphology, temperature field, microstructure and properties of aluminum alloy were systematically analyzed. The results show that the laser preheating and fluid cooling temperature control system can realize accurate preheating and eliminate heat accumulation, obtain a single track with a surface roughness Ra of 2.6 μm. High precision, high efficiency and stable LMD forming of AlSi10Mg overlap, block and thin-wall are realized. Preheating can improve the laser absorptivity, flatten the track and increase the grain size. The assist system and method can effectively solve the problem of insufficient forming stability of aluminum alloy LMD, and provide a new idea and process for forming quality control and molten temperature controlling.