A digital modulation method, a data transmission method and apparatus, and a communication system. The digital modulation method comprises: mapping to-be-modulated information comprising first information into a symbol having a first amplitude, and mapping to-be-modulated information comprising second information into a symbol having a second amplitude, wherein the probability of occurrence of the first information is less than the probability of occurrence of the second information, the sum of the probability of occurrence of the first information and the probability of occurrence of the second information is 1, and the first amplitude is greater than the second amplitude. Thus, the modulation performance can be guaranteed where the probabilities of occurrence of "0" and "1" are not equal.