The invention discloses a fault diagnosis and fault-tolerant control method for an ammonia gas input sensor of an SCR system of a diesel engine. The method comprises the following steps: an NH3 input concentration threshold value and a time threshold value are set at first, an NH3 input concentration measured value measured by an NH3 input concentration sensor is conveyed to an NH3 sensor fault diagnosis module, an NH3 input concentration estimation module estimates an NH3 input concentration estimated value, and the NH3 input concentration estimated value is conveyed to the NH3 sensor fault diagnosis module; and the NH3 sensor fault diagnosis module calculates an NH3 input concentration residual error according to the received signal, judges the working state of the NH3 input sensor through comparison with the input concentration threshold value and the time threshold value, and gives a final NH3 input concentration actual value. According to the method disclosed by the invention, a self-adaptive Unscented Kalman filter algorithm is adopted, the covariance of noise can be online calculated, and high adaptability to complex environment conditions is achieved; and moreover, the calculation load is lowered, so that a higher response speed is achieved, online fault diagnosis for the sensor of the SCR system is realized, and fault-tolerant control can be better carried out.