Abstract: For achieving precisely controlling shape and performance of components by additive manufacturing, scientific understanding is needed for thermal process. Taking laser metal-wire additive manufacturing (AM) under vacuum and using single-pass as an example, the thermal process was monitored based on infrared (IR) thermography. The effect of wire feeding speed on temperature field, thermal cycle, cooling rate was analyzed. The width of cladding layer and defect were studied based on IR thermography. The results show that the temperature monitoring was achieved. Along the length of cladding layer, the maximum temperature for monitoring point increased and then the cooling rate decreased. With wire feeding speed increasing, it led to the decrease of cooling rate for monitoring point at 1/4, 2/4, 3/4 of cladding layer. In addition, the width of cladding layer could be predicted and the location of defect could be located with the help of IR thermography analysis.