With the continuous development of aero-engine technology, the heat dissipation demand of high-temperature media inside the engine has increased. It is necessary to use the bypass air cooler to cool the high-temperature media with the low-temperature bypass air. The plate-fin heat exchanger is a relatively mature bypass air cooler solution. After long-term use, the plate-fin heat exchanger will accumulate dust on the surface and cause channel blockage. To evaluate the impact of channel blockage on the heat exchange performance and flow resistance performance of the heat exchanger, this paper uses the three-dimensional fluid–solid coupling method to evaluate the impact of channel blockage on the heat exchange performance and flow resistance performance of the heat exchanger under different working conditions. The bypass flow rate under different working conditions is dimensionless processed, and the variation curve of the pressure loss of the bypass fan section with the dimensionless bypass flow rate is obtained. Through the analysis of the simulation data, it can be concluded that after the cold side channel of the heat exchanger is completely blocked, the relative value of the flow resistance of the bypass fan section where the heat exchanger is located increases by 49%; for every 1% of the cold side flow area of the heat exchanger blocked, the heat exchange performance decreases by 0.76%.