Fouling in the membrane interior of engineered large sized charged nanopores can significantly impact the transmembrane ion transportation characteristic. Here impacts of fouling characteristics on the energy conversion performance are investigated. When the fouling located near the low concentration end, the shadowing effect that decreases the effective charged surface area plays a dominant role, which weakens the EDL overlapping degree, and lowers the energy conversion performance. Fouling also brings about blocking effect that narrows the ion transportation channel, and improves ion separation. When the fouling located near the high concentration side, the blocking effect could override the shadowing effect, leading to upgraded energy conversion performance. Due to the coupling of the shadowing effect and blocking effect, the electric power and energy conversion efficiency could overwhelm the performance with no fouling existed. The electric power and energy conversion efficiency under the fouling of r=25 nm are increased by 26.47% and 7.43%, respectively.