Because gas Cherenkov detectors can exclude most interference from secondary gamma rays, they have been the primary candidate for detection of fusion gamma rays. It is a goal worthy of long-term efforts to enhance the threshold contrast index used for evaluating the ability of gas Cherenkov detectors to exclude below-threshold interference. This paper presents a way of enhancing the index through ultraviolet reflection selective suppression of scintillation signals in the detectors. Both theoretical estimation and experimental verification demonstrate that the threshold contrast index can be enhanced by 5.5 times after applying this method. This provides possibilities of obtaining better fusion gamma-ray detection waveforms and higher confidence diagnostic information.