The invention relates to the technical field of engine waste gas treatment, in particular to a urea mixing device and an SCR (Selective Catalytic Reduction) system. The urea mixing device comprises a gas stream accelerator and a mixing pipe which is connected with the gas stream accelerator; the gas stream accelerator comprises a first conical pipe and a second conical pipe; one end of the mixing pipe is connected to the end, with a smaller pipe opening, of the first conical pipe; the second conical pipe is sleeved in the first conical pipe; the inner side of the second conical pipe is provided with a first gas passage; the outer side of the second conical pipe is provided with a second gas passage; and gas can enter the mixing pipe from above the gas stream accelerator through the first gas passage and the second gas passage. According to the urea mixing device and the SCR system, the mixing uniformity of a urea aqueous solution and waste gas is improved; and deposition of urea on the pipe wall of the urea mixing device is avoided.