The classification of conventional pyroclastic rocks in Chinese oil industry is often classified by the multiples of porosity. But unconventional oil and gas reservoirs, such as tight oil, have no uniform industry standard for tight reservoir classification. Generally speaking, oil has non-wettability and water has wettability for pyroclastic rocks. Therefore, oil and water will produce capillary pressure between the pore throat of tight reservoir and external environment which will promote oil and water flow. The flow rate is related to pore throat size, capillary pressure and buoyancy, which will cause the variation of oil and water saturation with porosity. The inflection point, extreme point or mutation point can be used as the boundary point of tight oil reservoir classification. In the end, the tight oil reservoir in the Sichuan basin can be divided into three kinds of tight reservoirs: fluid binding, capillary and oil-water balance.