A kind of theory surrounding with sub-optimum information system, the systematical non-optimum analysis method, has been presented, and thereby the practice courses and results of mankind are classified by their natures into three categories: optimum, sub-optimum and non-optimum, and also, the way of seeking out the maximum sub-optimum has been found out. It is considered that the non-optimum system information does not exclude the targets and the results of non-optimization in practice. The formation of non-optimum information system serves as the basis for existence of optimum system. Besides, the various characteristics and functions of the optimum system can be measured from the non-optimum information system. By summing the practice, this paper has also come at sub-optimum principle of the information system, established the conception of sub-optimum thresholds and put forward three theorems about sub-optimum parameters. Finally, according to the previous practice of optimization, kind of method has been developed to approach the maximum sub-optimum from minimum non-optimum.