The invention relates to a same-layer drainage device and a construction method thereof, and belongs to the technical field of drainage facilities. The same-layer drainage device comprises a ventilating vertical pipe, a drainage vertical pipe and a floor drain which are located on a floor. The ventilating drainage pipe is communicated with the drainage vertical pipe through an inclined three-way joint. The drainage vertical pipe is communicated with a same-layer drainage pipe through a vertical three-way joint. A drainage opening and a ventilating opening are formed in the middle, in the height direction, of the drainage end of the floor drain. A middle partition plate is arranged in the middle of a box body. A water sealing plate is arranged between the drainage end and the middle partition plate. A slope facing the drainage end is arranged at the bottom of the box body. The drainage opening is communicated with the drainage vertical pipe through a horizontal connecting pipe. The ventilating opening is communicated with the ventilating vertical pipe through a climbing connecting pipe. The defect that when a traditional floor drain is installed, a drainage pipe is located on the lower layer and interferes with construction of a ceiling on the lower layer is overcome, the edge of an upper opening of the floor drain is flush with or slightly lower than the floor, the drainage requirement is met, a waterproof layer of the floor is not damaged, the drainage silencing effect is remarkable, and bacterium breeding and odor are avoided.