Abstract: The formation of continuous melt tracks through a low-power beam for powder bed fusion is hindered by the inadequate thermal conductivity exhibited by a loose powder bed.To increase the depth of energy penetration during the solar convergent molten lunar soil printing process，an in-situ observation study of the melt pool formation process was conducted when a large-sized spot melted a simulated lunar soil powder bed.To simulate the concentrating spot of sunlight，the laser spot was expanded to millimeter scale and impacted directly on the surface of a thick powder bed lacking a substrate.Using a technique of high-speed imaging，the development of melting droplets was observed.The partially melted surface powder was discovered to exhibit a large-scale interparticle cooperation referred to as the “wrapping” process.In this process，the powder bed's surface layer was rolled up，separated from the powder bed，and joined to the molten droplet，resulting in a growth that is discontinuous.The growth law of melting droplet size was investigated and compared with experimental results based on the energy conservation relationship between the spot and the heating of the powder bed.The results indicate that melt droplets generated by a low-power，largespot laser cannot extend beyond the range of the spot.The higher the power density，the less complete the powder melting while a wrapping process occurs，which increases the energy efficiency of this operation.The utilization of the wrapping mechanism effectively overcomes the constraint posed by the low thermal conductivity，particularly in the context of lunar soil powder bed fusion technology.This mechanism enables the direct printing of melting tracks on substantial powder beds.

Key words: lunar soil, additive manufacturing, solar powder bed fusion, insitu observation, spot size, molten droplet radius