The invention provides a system for detecting water seepage of a porous medium insulation plate and a method of the system, belonging to the technical field of electrical capacitance tomography. The system comprises a parallel capacitance array sensor, a capacitance measuring module and an upper computer which are sequentially connected; the sensor comprises excitation detection electrode plates respectively positioned at the inner side and the outer side of the porous medium insulation plate to be detected, and a to-be-tested cabin metal shell taken as a common electrode; the excitation detection electrode plates are respectively provided with an electrode array, and the electrode arrays form electrode pairs respectively with the common electrode; the capacitance measuring module is usedfor sequentially acquiring transverse and longitudinal two groups of capacitance information of the electrode pairs; the upper computer is internally provided with an electric field sensitivity distribution calculation module and an image reconstruction data analysis module. According to the method, the water seepage position and water seepage degree of the porous medium insulation plate are determined by sequentially carrying out data acquisition, information processing, image reconstruction and data analysis. The system and the method have the characteristics of convenient operation, high detection efficiency and accuracy, deep detection depth, large amount of acquired information and strong expansibility.