The invention relates to a synthesis method for organically synthesizing an intermediate N-allylthiourea. The synthesis method mainly comprises: adding 4-5 mol urea, 3 mol allyl alcohol, 12 L of a potassium bromide solution and 1.5 mol alumina powder in a reaction container, controlling the stirring speed at 90-110 rpm, heating the solution to a temperature of 80-90 DEG C, maintaining for 100-130min, carrying out pressure reducing distillation, cooling the solution to a temperature of 10-15 DEG C, precipitating to obtain a crystal, washing with an ether solution, washing with a dimethylaminesolution, re-crystallizing in a methanol solution, and dewatering with a dewatering agent to obtain the finished product N-allylthiourea.