In the process of microsecond and millisecond pulsed discharge in water, a liquid low-density region will be formed by the effect of Joule heating. Then, thermal fluid is formed. Thermal fluid can influence the flow field and temperature distribution; and thus, it will affect the development of discharge. In this paper, the schlieren technology is used to observe the development of thermal fluid. The temperature field around the thermal fluid is calculated by the quantitative schlieren method. Finite element analysis is used to simulate the development of thermal fluid. Results show that the development of thermal fluid involves the coupling of electric fields, flow fields, and thermal fields. When there is an electric field, the movement of thermal fluid is mainly driven by the electric force, and it conforms to electrohydrodynamics. When the electric field disappears, the movement of thermal fluid is mainly driven by pressure. Due to Rayleigh-Taylor instability in the thermal fluid's head, the thermal fluid takes on a mushroom shape. The change in the thermal fluid's development direction on the side of the electrode leads to a short increase in the equivalent resistance of the gap. The study of thermal fluid is helpful to qualitatively understand quasi-DC discharge in water.
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