The invention relates to a vehicle-mounted mobile vehicle washing device and method. The vehicle-mounted mobile vehicle washing device mainly comprises a transporting vehicle, a box type vehicle washing room, a water tank group, a pump unit, a sewage circulating system, a control system, a pipeline system, a washing medium unit and an intelligent vehicle washing equipment unit. The transporting vehicle reaches a designated place, the box type vehicle washing room is pulled open, a track bracket is mounted and the direction of intelligent vehicle washing equipment is adjusted (if needed), waterand electricity lines are connected, a supporting inclined plate is placed, a vehicle runs into the box type vehicle washing room, and the intelligent vehicle washing equipment can wash the vehicle.When operation is finished, the vehicle can be transported to the next location by folding the box type vehicle washing room, detaching the track bracket and adjusting the azimuth of the intelligent vehicle washing equipment (if needed). According to the vehicle-mounted mobile vehicle washing device, the dependence of vehicle washing services on a fixed shop is eliminated, the vehicle washing costis reduced, door-to-door service is provided, vehicle washing sewage can be recycled, environment-friendly significance is huge, mobile vehicle washing becomes a brand new industry, and social employment and the development of related industrial chains are promoted.