The invention discloses a device and method for controlling a urea injection system of a diesel engine. The device comprises a working condition parameter acquiring device, a control-amount determining device and a driving signal determining device, wherein the working condition parameter acquiring device is used for acquiring working parameters which are relevant to the urea injection system; the control-amount determining device is coupled with the working condition parameter acquiring device and is used for determining the control amount for controlling the urea injection system according to the working condition parameters, a target value of urea pressure in a urea buffer chamber and a control model which is designed based on a physical model for representing the urea injection system, and the control-amount is the rotation speed of a urea pump driving motor; and the driving signal determining device is coupled with the control amount determining device, and is used for determining the driving signal for driving the urea pump driving motor according to the determined control-amount. According to the invention, the implementing method is suitable for working processes under anyworking condition, more accurate urea injection pressure and rapid response can be realized, the deviation between urea pressure and the target value of the urea pressure is lowered, the calibrated work amount is reduced, and the function of the system is improved.