The invention provides polyurethane for a single component coating and a preparation method of the polyurethane. The preparation method comprises the following steps: S1, enabling binary isocyanate and a sealing agent to react so as to obtain a first mixture, wherein the mole ratio of the binary isocyanate to the sealing agent ranges from (0.9:1) to (1:1); S2, adding water into the first mixture, and performing chain expansion reaction, thereby obtaining the polyurethane. In the preparation process, by controlling the mole ratio of the binary isocyanate to the sealing agent and/or the mole ratio of the binary isocyanate to the water, and controlling the reaction temperature, the mass of the polyurethane can be effectively controlled. Compared with the prior art that the polyurethane preparation process is complex to control, rich in reagent type and high in cost, the polyurethane for the single component coating and the preparation method of the polyurethane provided by the invention are small in type of agents and low in cost.