Thanks to its quick response, high precision and simple structure, piezoelectric micro-jet has been applied in various industrial fields. Especially in 3-D printing and surface coating whose accuracy has an important relationship with the spreading characteristics of the ejected droplets. A piezoelectric microjet, which can be used for 3-D printing and coating, is presented in this paper. The influences of excitation parameters on the spreading characteristics of ejected droplets are analyzed by a proposed tree dimension multi-physical coupling analysis method, in order to improve the ejection accuracy of the piezoelectric microjet. The spreading processes from the initial forming to the complete spreading of droplets are given. Based on the obtained results, the recommended adjustment schemes of excitation for improving the droplet spreading precision are given. Besides, the adjustment methods of ejecting distance and height are given to avoid the mutual interference during the process of droplet spreading. Experiments are carried out, the proposed coupling method and analysis results are proved to be effective.