Abstract:Risk Matrix Method (RMM) is widely used in engineering field since it is succinct and practical to perform, while the validity of its assessment results has also been challenged since its appearance. By illustrating the connotation of risk in Risk Matrix Method and the similarity between Risk Matrix and Covariance Matrix, a general retrospection on the origin of RMM is performed. The basic mathematical model of Risk Matrix is introduced by adopting classification function, elucidating some theoretical issues of RMM. According to the accuracy issues of risk assessment, the notion of uncertainty is proposed as an evaluation index. The meaning of RMM uncertainty in engineering application is clarified. Based on the mathematical model, appearance of RMM uncertainty, sourcing from Risk Matrix establishment process, risk ties and the matrix evaluation form, are discussed. In the end, a general review on how to recognize and control the corresponding uncertainties properly and effectively is delivered and the future development directions are provided.