The invention relates to a method for obtaining pure hydrazine hydrate by isolating and purifying from solution containing hydrazine. The preparation method is that gas-phase acetone and the hydrazine which is contained in oxidation liquid are directly reacted and stripped under the boiling condition; or liquid-phase acetone and the solution are reacted first and then gas stripping is carried out, after the solution is hydrolyzed, the pure hydrazine hydrate is obtained; and the obtained product is sent into a general hydrolyzing tower, in which hydrolysis is carried out to produce the pure hydrazine hydrate solution, and acetone produced by the hydrolysis is sent back to a system for recycling, wherein, the mole ratio of the acetone and the hydrazine is 2 to 3.6 and the reaction temperature is 100 DEG C to 130 DEG C. By adopting the invention, the hydrazine produced by urea-chlorine method can be stripped by acetone azine (CH3)2C=N-N=C-(CH3)2, which causes the recycling of the inorganic materials such as sodium carbonate, sodium chloride, and the like, in the solution to be possible, thereby thoroughly solving the problem of waste water condensation of the ADC processing technique in the urea-chlorine method.