The invention provides a washing machine and an overflow device thereof. The overflow device comprises a water receiving box arranged on the lower portion of an outer drum of the washing machine, an overflow port which communicates with the water receiving box is formed in the outer drum, and an overflow plug structure for opening and closing the overflow port by utilizing the pressure of the received washing water is arranged at the position of the overflow port. By arrangement of the overflow plug structure, the washing water in the outer drum is increased, the water level is increased, andthe pressure of the washing water acting on the overflow plug structure is increased; when the pressure of the washing water acting on the overflow plug structure is greater than the set pressure of the overflow plug structure, the overflow plug structure is pressed to move downwards, so that the overflow port is opened, the washing water in the outer drum flows out of the outer drum through the overflow device, the washing water in the outer drum is reduced, and the pressure of the washing water acting on the overflow plug structure is reduced; and it ensures that the pressure of the washingwater in the outer drum is kept within a certain pressure, the problem caused by over-high water level in the outer drum is avoided, and the safety is improved.