The invention relates to a fault judgment method and device and differential protection method and device for a flexible DC transmission and distribution system. The fault judgment method comprises the steps of carrying out corresponding current sampling according the type of equipment requiring protection in the flexible DC transmission and distribution system; and judging that a fault of the equipment requiring protection occurs if the conditions are met as follows: (1) the sum of products of absolute values of differential current of sampling current of a first set number of continuous sampling points and a program execution cycle of a protection device is greater than a first set current threshold; and (2) the absolute values of differential current of sampling current of a second setnumber of continuous sampling points all are greater than a second set current threshold. A fundamental wave effective value is extracted without Fourier transformation, so that the fault judgment device has a higher fault detection speed, the possibility is provided for providing relatively early outlet protection time, and the fault judgment method and device are more suitable for the flexible DC transmission and distribution system with a high fault current development speed.