An exposure-control system and an associated exposure control method are provided. The exposure-control system includes: an image capturing unit configured to capture a long-exposure image and a short-exposure image with a first exposure value and a second exposure value, respectively; and a processor, configured to calculate histograms of the long-exposure image and the short-exposure image, and calculate an exposure ratio according to the calculated histograms, the first and second exposure values, wherein when the exposure ratio is smaller than a first threshold, the processor switches a current exposure mode to a low dynamic range mode. When the exposure ratio is larger than a second threshold, the processor switches the current exposure mode to a high dynamic range mode. When the exposure ratio is between the first threshold and the second threshold, the processor does not switch the current exposure mode.