Abstract: Microplasma jet has higher plasma density and processing accuracy, and is extensively applied in nanomaterial preparation, precision machining and surface etching. However, the methods and mechanisms for controlling the characteristics of atmospheric pressure microplasma jet in open environment are not yet clear. Therefore, we built a system of atmospheric pressure microplasma jet and in-site diagnosis, and Ar jets with diameter of 50~200 μm were achieved. Moreover, the influences of capillary inner diameter, gas flow rate and applied voltage on the discharge characteristic, jet length, gas temperature and electron density of microplasma were analyzed through voltage and current waveform, jet image and emission spectrum. The results show that decreasing the capillary diameter can reduce the gas temperature and jet length, but raise the breakdown voltage and electron density. When plasma jet is in the criticality of laminar and transition modes, the performances of microplasma jet are optimal. Increasing the applied voltage can raise jet length, gas temperature and electron density. The gas temperature and electron density are determined by the jet specific surface area and input energy density, respectively. The radial diffusion of active particles reduces the jet length and electron density. This work reveals the regulation laws and mechanisms of atmospheric pressure microplasma jet characteristics in open environments, and is of significance for the application of microplasma jet.