In the aerospace system, there are many reliability parameters to describe the reliability characteristics of products, but most of them are based on continuous reliability parameters in continuous time, and there are fewer descriptions of discrete reliability parameters that are defined as reliability parameters in discrete time, such as discrete failure probability. In order to make up for the shortcomings of reliability theory in this aspect, the mathematic relationship of discrete reliability parameters are studied from the perspective of discrete time, including the mathematical conversion formula of discrete failure probability and discrete failure rate, the mathematical of calculating discrete reliability by discrete failure rate, and the mathematical conversion formula of mean work times to failures and discrete failure probability. These works can provide the mathematical theory basis for the conversion calculation of reliability parameters and indicators.