As a new type of carbon nitride material with high nitrogen content, g-C3N3 has promising application potential in the fields of photocatalysis, electrochemistry and energy storage. In this study, we found that g-C3N3 can be dissolved in concentrated sulfuric acid at room temperature to form a true thermodynamic solution for the first time, and the g-C3N3 can be reversibly precipitated successfully by adding deionized water to the solution. The solution with a high concentration of 300 mg ml-1 exhibits good stability even at -40 °C. The dissolution phenomenon of g-C3N3 in concentrated sulfuric acid and the solution stability at low temperature will pave the way for its potential applications.