The invented preparation method puts urea in a specific amount of H2O2 (molar ratio being urea:H2O2=1:0.1-1) and a little of water, uses Pt compound or Cu compound or PtCu compound as catalyst (dose of catalyst is 0.05-1% of urea weight) and heats on household microwave oven or electric stove to make them react rapidly and become cyanuric acid which is dissolved in hot water and cooled and stayed still to obtain the two crystal water bearing cyanuric acid. said method can complete the reaction in 2-10 min, the operation is very convenient, safety and reliable and features low energy consumption and cost but high yield.