This paper studies the surface quality of tool steel with high-speed wire electrical discharge machining (WEDM) method, which is conducted in gas to improve the accuracy of finish cut, and compares the surface quality in atmosphere and in emulsion dielectric. Experiment results showed that WEDM in atmosphere offers advantages such as better straightness and higher material removal rate. With the growth of wire winding speed, the removal rate and straightness error will increase significantly either in atmosphere or in liquid. Along with the feed increasing, the discharge gap lengthes decrease successively under the both media conditions, and the straightness is obviously improved in atmosphere. Regardless of the use of atmosphere or emulsion, an optimum feed will exist, which makes the roughness the lowest and the removal rate the highest.