The invention provides a plasma-melting electrode electric arc dynamic composite welding device and a welding method thereof. A plasma arc welding gun is combined with a melting electrode electric arc welding gun to obtain a composite welding gun; the two electric arc welding guns are respectively connected with a negative pole of a direct-current pulsed plasma arc welding power supply and a positive pole of a direct-current pulsed melting electrode arc welding power supply and are also connected with a negative pole and a positive pole of an argon arc welding power supply (a TIG power supply); and a negative pole of the plasma arc welding power supply and a positive pole of the melting electrode arc welding power supply are connected with a workpiece. In order to prevent electromagnetic interference between a plasma arc and a melting electrode electric arc during welding, a pulsed coordination control method is used, namely, when melting electrode electric arc current waveform is a pulsed basic value pilot arc, the plasma arc is a pulse peak value, and welding is carried out; and when plasma arc current waveform is a pulse basic value, the melting electrode arc is pulse peak value welding current, and so forth. By the plasma-melting electrode electric arc dynamic composite welding device and the welding method thereof, the problems that electromagnetic interference between electric arcs is large, parameters are adjusted complexly, and electrodes are burnt down severely are solved.