The invention provides a synthesis method of a urapidil pharmaceutical intermediate 1,3-dimethyl-6-(3-hydroxpropyl)aminouracil. The synthesis method comprises the following steps: adding 0.61mol of 1,3-dimethyl-6-hydroxluracil (2), 0.72mol to 0.75mol of 3-amino-1-propanol and 0.16mol of a sodium sulfite solution into a reaction container provided with a stirrer, a thermograph and a reflux condenser; controlling a stirring speed to be 130rpm to 160rpm; raising the temprature of a solution to be 150 DEG C to 160 DEG C and reacting for 90min to 120min; lowering the temprature of the solution to be 60 DEG C to 65 DEG C and adding 230ml of cyclohexane; refluxing for 30min to 50min and filtering; lowering the temperature of filtrate to be 5 DEG C to 9 DEG C; standing for 30h to 35h; separating out a crystal and filtering; washing the obtained crystal with a salt solution and then washing with acetonitrile; dehydrating the crystal with a dehydrating agent; recrystallizing in isopropyl alcohol to obtain a white crystal, namely the 1,3-dimethyl-6-(3-hydroxpropyl)aminouracil.