Disclosed in the present invention is a method for safely preparing pimavanserin and a tartrate salt thereof using triphosgene. The synthesis routes comprise (1), (2), (3), (4) and (5). The raw materials used in the method of the present invention are safe, and have low cost, effectively reducing the production cost. The reaction conditions of the method of the present invention are mild, the use of phosgene, having high toxicity and being difficult to operate, can be avoided, and the method is easy to implement industrially.