The invention relates to a parking lot, and especially relates to a parking lot built on a viaduct. The parking lot built on a viaduct is characterized in that the viaduct is arranged under the parking lot, and the top surface of the viaduct comprises a multilayer garage frame, an elevator in the center position of the frame, spiral lanes, parking spaces distributed in a diverging manner, parking space detectors, a parking space display, timing card readers and parking place road gates. The center of the parking lot is provided with a circular platform, and the circular platform is a platform serving for drivers and conductors; spiral stairways are arranged outside the central circular platform, low-layer drivers and conductors can walk up and down the stairs, and safety channels are provided for the parking lot, so the parking lot and the viaduct are safe; the parking lot is provided with an ascending one-way road and a descending one-way road, so the ascending one-way road and the descending one-way road of the parking lot are not interference, and convenience is realized; and an arc-shaped reversing lane used for connecting the ascending one-way road with the descending one-way road is arranged every 90 DEG corner every layer, so the disadvantages of realization of lane change through multiple-cycle motion and difficulty in lane change of previous spiral parking lots are solved.