With the discovery of more and more low-pressure reserviors at home and abroad, the mechanisms of low pressure and its relation to oilgas reservoir forming in sedimentary basin have been drawing increasingly attention. Through investigating the corresponding references, this paper systematically analyzes the present research situation, problems and development trend of the origin of subnormal pressure. The origin of subnormal pressure can be concluded mainly into four aspects: (1) pore space increase; (2) pore fluid volume decrease; (3) piezometric surface decline; (4) original pressure preservation. It also indicated the basic reason of low pressure formation is the unbalance of supplying and discharging of pore fluid. The studies of formation mechanism of low pressure and its relation to oil-gas reservoir forming have made progress. The quantitative simulation and evaluation of controlling factors of low pressure and the pressure evolution of low pressure reservoir are the major research subjects in future.