Abstract: Objective To investigate the effect of process parameters on vertical additive forming in twin-body plasma arc additive manufacturing , the width and height of single-pass cladding layer were used as evaluation indexes. Methods Based on the construction of vertical twin-body plasma arc additive manufacturing system, and the effect of process parameters such as the ratio of wire/total current , plasma gas flow rate and end position of wire on cladding layer forming was studied by single factor variable method. The correlation mechanism of process parameters and substrate heat input with cladding rate was analyzed. Results The results showed that when the total current remained unchanged, the formation of cladding layer could be improved significantly with the increase of wire current, and the width of cladding layer decreased and the height of cladding layer increased. With the increase of plasma gas flow rate, the width of cladding layer increased and the height of cladding layer decreased due to the increase of plasma flow force. When the wire feeding speed increased, the position of the wire was close to the arc center, the width of cladding layer decreased and the height of cladding layer increased as the mass transfer increased and the heat transfer decreased. When wire current increased, the size of cladding layer changed by changing the transmission of heat, mass and force in the system. When the substrate current increased, the size of cladding layer changed by changing the heat and force transmission. Conclusion When the total current remained constant, the heat input to the substrate could be reduced and the cladding rate could be increased by increasing the wire/total current ratio, decreasing the plasma flow rate, and moving the end of the wire away from the substrate, or directly increasing the wire current to increase the cladding rate , so as to achieve efficient additive manufacturing. This study was expected to provide theoretical guidance for applicating the twin-body plasma arc additive manufacturing process in the field of spatial position additive manufacturing.