A device and a method for testing ash deposition characteristics of a flue gas waste heat exchanger are disclosed, the device includes a test section heat exchanger, a flue gas circulation loop, a water circulation loop and a test system, and the flue gas circulation loop, the water circulation loop and the test system are respectively connected with the test section heat exchanger. Gas-side high temperature dust-containing flue gas of the test section heat exchanger in the flue gas circulation loop is led from a boiler, flows through the exchanger, and sent back to the boiler, water-side water supply of the test section heat exchanger in the a water circulation loop is directly from boiler water supply; the testing system can be used to calculate the fouling resistance of the test section heat exchanger by monitoring and calculating at any time the decrease trend and the declining quantity with time of heat transfer coefficient of the test section heat exchanger so as to evaluate the ash deposition characteristics of a heat exchanger tube. The types and the layout mode of testing tubes can be replaced freely without influence on the operation of the boiler, the testing method can be visualized, and monitoring of the ash deposition condition of a test section can be facilitated. The device overcomes human errors caused by measuring of the ash deposition weight in the prior art and the defects of the inconvenience of installation, dismounting and replacing, and the like of the heat exchange tube due to installation of the test device in the boiler.