Abstract: Corona discharges characterized by much lower current densities are often generated in a strong atmospheric electric field. What's more, corona discharges are very important because the corona charge layers can change the distribution of the electric field near the ground and affect conditions of the initiation and development of an upward leader near different tall objects. Therefore, corona discharges have attracted the attention of scholars at home and abroad, especially in the onset corona field. However, how to effectively show corona emission threshold is still a difficult problem in the study of atmospheric electricity.In order to obtain the electric field strength at the tip of metal in the case of corona discharge, the onset corona voltages must be obtained by laboratory test for different heights, shapes and materials of metal tips which are arranged on the middle of lower plate of two parallel metal plates. Dimensions of two horizontal plates are 1.2 m by 1.2 m and the distance between them is 1 m. Then environmental threshold electric fields are calculated for different heights, shapes and materials of metal tips. The two-dimensional Poisson equation is solved using finite element method and the corona emission thresholds at the tip of different types of metal tips are obtained.Effects on threshold electric fields by the height, material and shape are discussed, respectively. The following conclusions can be drawn from the experimental study. First, the environmental threshold fields are basically a linear decrease with heights of metal. The environmental threshold fields decrease first and then increase when metal tips are becoming sharp increasingly. Second, the height and shape of mental tips have no effects on corona emission threshold at the tip of metal tips. The corona threshold at the tip of mental tip is a constant. Finally, the corona triggering threshold is determined which is E_c=158.75 kV·m^-1 and the fitting function between corona emission threshold and spatial resolution is given. It will provide a reference for the judgment of starting moment of corona discharge for the future numerical simulation of corona discharge.