The invention relates to an on-board mobile car washing device and a method. The on-board mobile car washing device mainly comprises a transportation vehicle, a water tank group, a pump unit, a sewagecirculation system, a control system, a pipeline system, a cleaning medium unit, an intelligent car washing equipment unit, a combined car washing house and a folding sewage collection tank. When a transport vehicle reaches the designated place, the folding sewage collection tank is unfolded and the combined car washing house is built, a steerable bracket is rotated (if necessary), an extended track is installed, the extended track is connected to the support at the other end and fixed, water and electricity lines are connected and a support sloping plate is placed, and the intelligent car washing equipment can clean the vehicle entering the car washing house. After completion of the operation, the bracket and the car washing house are dismantled, the sewage tank is folded onto the transport vehicle or hung in the rear of the vehicle, and then the car washing device is transported to the next place. The car washing device makes the car washing service get rid of the dependence on thefixed stores, reduces the car washing cost, provides door-to-door service, the car washing sewage can be recycled, and the car washing device has great environmental protection significances, and makes the mobile car washing become a brand-new industry to drive the development of social employment and industrial chain.