Abstract: Through testing the shape of micro-plasma arc,the dispersion of arc current and the ratio of weld width against the fluctuation of arc length, the authors put forward that neither the constant-current power source nor the hyperbolic charscierislic power source could make the heat input to the anode (workpiece) constant under the fluctuation of arc length. Only when the arc is intensely constricted by the mixed gas, the power source with hyperbolic external characteristics would then be slightly better. The main cause of the variation in the heat input with arc length is that the arc itself has current dispersion. This paper provides a theoretical basis for developing the optimum power source for micro plasma arc welding.