Abstract: Atmospheric pressure plasma jet has large application prospects in many fields such as surface treatment, biomedicine and so on, but its application is limited because of the complexity of the airflow control system. In this paper, based on a traditional single-needle jet structure and appropriate configuration design, a diffuse plasma jet without airflow was produced, and the influences of tube diameter and air gap spacing were investigated. Moreover, the development process of the diffuse jet was also discussed. It is shown that the diffuse jet is generated by the interaction of external electric field, surface charge and space charge. The length of jet and the current amplitude decrease with the increase of air gap spacing. In the case of small tube diameter, the diffuse jet presents a gradual process of "corona → jet generation → jet development → channel formation" with the increasing voltage amplitude, and the negative discharge could be observed. The diffuse jet is generated as a sudden process of "corona → channel formation" under a large tube diameter condition. The two unique phenomena and the development processes of diffuse jet are explained based on the dielectric surface process theory. A simple plasma jet device was provided in this paper which can be helpful in improving the application field of plasma jet.