The invention relates to the field of vehicle emission, in particular to a diagnosis method for an emission system. A diagnosis method for desorption flow of a high-load desorption pipeline of a carbon tank comprises the following steps: selecting a vehicle with an intact desorption pipeline as a standard vehicle, starting the vehicle, opening an electromagnetic valve of the carbon tank, and closing a stop valve of the carbon tank; waiting for a certain time, and acquiring the vacuum degree in an oil tank; testing repeatedly under each residual fuel amount to obtain a function of the vacuum degree and the residual fuel amounts; and when the diagnosis is performed in actual travelling of the vehicle, comparing a vacuum degree threshold value obtained according to the actual residual fuel amount and the function with the actual vacuum degree to realize the diagnosis of the high-load desorption pipeline. In step S1, the vehicle is started and kept in an idle state. According to the diagnosis method, the diagnosis of the desorption flow of the high-load desorption pipeline is completed by using an existing sensor and an existing executor; and a pressure sensor is not required to be mounted in a Venturi pipe, so that the cost is reduced, meanwhile, the matched wiring harness layout of the sensor is also simplified, the system structure is simplified, and the requirement on light weight of a compact vehicle is met better.