The invention relates to the technical field of drainage engineering, and particularly relates to a construction method of a cast-in-place drainage ditch. The construction method comprises the following steps that a groove is excavated; the groove is leveled; a bottom die is installed, the bottom die is divided into a left part and a right part, the two parts of the bottom die are fixed to the bottom of the groove, and the bottom of the groove supports the bottom surfaces of the two parts of the bottom die; concrete is poured, specifically, the concrete is poured between the two parts of the bottom die, and the concrete flows downwards along the side walls of the two parts of the bottom die and is converged between the two parts of the bottom die; a pressing die is installed, specifically, the pressing die squeezes the concrete at the bottom of the bottom die, so that the concrete is used for filling between the side wall of the bottom die and the side wall of the pressing die, and shaping is carried out; the die is removed, specifically, after a drainage ditch main body starts to be hardened, the pressing die is removed, and after maintenance is completed, the bottom die is removed; and the groove is backfilled, specifically, after the die is removed, slurry supplementing or sand and stone backfilling is carried out at the bottom of the drainage ditch main body, and then the groove is backfilled and compacted. According to the construction method, the construction efficiency of the urban cast-in-place drainage ditch is improved, and the construction steps are simplified.