The invention belongs to the technical field of organic synthesis, and more specifically relates to 4-ethylamine(2-carboxyl)-9H-pyrimido[4,5-b] indol, and a preparation method thereof. According to the preparation method, 4,6-triethoxycarbonyl-1,3,5-triazine and 2-aminoindole are taken as raw materials, an intermediate product is obtained via inverse electron demand diels-alder reaction, and a target product is obtained via ester group hydrolysis selective decarboxylation and amide condensation reaction. The preparation method is simple; target product yield is high; the preparation method is suitable for industrialized production; and the prepared 4-ethylamine(2-carboxyl)-9H-pyrimido[4,5-b] indol possesses potential application value in treatment of cardiovascular diseases, nephrosis, and diabetes.