The capillary discharge triggered by a pulse source can produce a certain length of plasma jet. In this paper, the physical process of the capillary discharge jet is analyzed, and it is pointed out that the capillary plasma-jet length is significantly affected by the expansion pressure caused by the arc discharge in the capillary chamber. The greater the pressure in the capillary chamber is, the longer the jet length. The experimental setup of the capillary discharge is established in atmospheric pressure air. The influence of the surface distance along the capillary wall, the diameter of the capillary cathode, and the length of the cathode tip on the plasma-jet length is studied under a specific trigger pulse. The experimental results show that the greater the deposited energy density in the capillary chamber is, the longer the plasma jet length. As the surface distance increases, the energy deposited in the arc channel increases first and then tends to be saturated. There is an optimum surface distance to maximize the energy density in the capillary chamber, while the plasma jet length is the longest.