The invention discloses a passive self-walking pipeline inner wall spray-coating car and an application method thereof. The spray-coating car comprises a hydraulic motor, a first chassis and a second chassis, wherein the first chassis and the second chassis are arranged at the two sides of the hydraulic motor; an energy accumulator and an unloading machine are arranged on the first chassis and are connected to the hydraulic motor; a rotary disc is arranged inside the hydraulic motor; a spray-coating pipe is arranged at the sidewall of the hydraulic motor; a second wheel group and a third wheel group are arranged at the bottom of the second chassis; a driving shaft is arranged between the second wheel group and the third wheel group; a first bevel gear is arranged at one end of the driving shaft and is meshed with a second bevel gear of the second wheel group; a belt wheel is arranged at the other end of the driving shaft and is meshed with one end of a belt; the other end of the belt is connected with the hydraulic motor; a high-pressure paint can be input to the hydraulic motor by virtue of the energy accumulator; the high-pressure paint flows through the rotary disc inside the hydraulic motor and is sprayed out via the spray-coating pipe so as to realize spray-coating of a pipeline; meanwhile, torque is generated to drive the hydraulic motor to rotate in order to drive the spray-coating car to automatically walk inside the pipeline; therefore, uniform spray coating can be realized, no electric power is needed for driving, and the spray-coating car is safe and labor-saving.