Mercury injection method is very important in research of pore structure of rocks. Conventional mercury injection method can be used to determine rock’s pore size and its distribution. Constant-speed mercury injection method expanded the functions of conventional mercury injection method, and can determine pore’s and throat’s parameters of rocks. However, according to the study of this paper, the function expanded by the constant-speed mercury injection method is the misuse of the “noise” of pressure data. Pores of rocks build an interconnected 3D pore net. There are no concepts of pore body and pore throat. Every pore is connected with surrounding big pores and small pores. Mercury would not enter throats before entering pore bodies, which choose the bigger pores first and then the smaller pores to enter. The sawtooth-like curve of mercury injection is not the reflection of pore-throat structures of rocks, which is the pressure fluctuation caused by the mechanical process of mercury injection apparatus. The pressure fluctuation of oil wells in production process is also not caused by pore-throat structures of reservoir rocks.