The invention discloses a method for efficiently preparing a monocyanamide solution, and is used for solving the problems of high equipment investment, high energy consumption, long production period, high production cost, high requirement on concentration of carbon dioxide, high content of calcium ions and incomplete filtration of cyanamide waste residue of an existing production process of the 50% monocyanamide solution. The method comprises the steps of performing a hydrolytic decalcification process step, adding phosphoric acid to regulate pH value, and performing microporous filtration, and evaporation concentration to obtain a monocyanamide product. The monocyanamide solution is prepared by improving a gas inlet way of kiln gas and applying a microporous filtration method, an ion exchange resin decalcification technology and a rotary thin film scraper steam method. Compared with a domestic traditional process, by adopting the method, the requirement of the traditional process on the concentration of carbon dioxide is reduced, the content of the calcium ions in the monocyanamide product is strictly controlled, the product quality is improved, the production period is shortened, the energy consumption is reduced, and the production cost is reduced, so that the method is suitable for industrial large-scale production and has good social and economic benefits.