The archimedes spiral cord modulating chopper's exposure efficiency was theoretically analyzed and numerical simulated, and its impact on the following signal processing was obtained. Results show that chopper's exposure efficiency directly affects detector pixels' exposure time and incident radiation, and affects the detector pixels' effective accumulated charge's uniformity with detector's signal readout time coordinately. Modulating synchronization locating point should be added in the chopper, which can make thermal imager be compatible with different exposure efficiency chopper. The effect of the chopper's exposure efficiency should be synthetically considered when the whole thermal imaging system and its electronic processing system are designed.