The invention discloses a joint preparation method for morpholine and imidazolidinone. The joint preparation method comprises the following steps: uniformly mixing 100 weight parts of diethanol amine, 29 to 58 weight parts of cyanuric chloride and 700 to 790 weight parts of water; reacting for 15 to 16 hours at the temperature of 85 to 95 DEG C; distilling under reduced pressure to remove water; drying distilled residues to obtain a white solid; performing pyrolysis on the white solid to constant weight in inert gas flow at the temperature of 258 to 270 DEG C; simultaneously, cooling and liquefying all pyrolysis gas which is exhausted from the inert gas flow, and introducing the pyrolysis gas into fractionating equipment in a centralized manner; fractionating to obtain morpholine distillate at 128 to 129 DEG C and imidazolidinone distillate at 258 to 265 DEG C; finally, purifying the two distillates and respectively obtaining purified products of the morpholine and the imidazolidinone. According to the joint preparation method for the morpholine and the imidazolidinone, the morpholine and the imidazolidinone can be prepared in a joint manner; the total yield of the morpholine and the imidazolidinone reaches 50.6 percent or above; moreover, a pyrolysis residue by-product with a flame retardant function can also be obtained synchronously, and the yield of the pyrolysis residue by-product is 7.8 to 9.4 percent.