The edge-localized modes (ELMs) are often excited in an H-mode plasma, and they are helpful for cleaning the H-mode plasma to sustain a steady state for a longer time by controlling plasma density and exhausting impurities, but energy and particles carried by ELM burst will badly damage the first-wall of fusion device, thus the characteristics of and the control and mitigation of ELM are studied necessarily prior to the basic operational regime operating on ITER. ELMs of different perturbation amplitudes are observed experimentally on HL-2A tokamak. The frequency of small perturbation amplitude ELM decreases with the increase of net heating power, and it is about 300-400 Hz, and energy loss induced by per ELM is usually less than 3% of the plasma energy. The small ELM is type Ⅲ ELM. While for large (type-I) ELM, besides that the energy loss induced by an ELM is generally more than 10%, they also exert an obvious perturbation on other plasma parameters, such as plasma current and electron density, and the tELM may be longer than 30 ms. ELM precursors are poloidally asymmetric, which can be measured by Mirnov probes on the low field side, but not on the high field side; the frequency of ELM precursors is about 45 kHz, and the longest precursors last approximately 10 ms prior to the ELM bursts.