To investigate the evolution characteristics of the air pocket during the geyser process, a numerical simulation of the release of an entrapped air pocket in a drainage pipe system under double-sided inflow conditions was conducted. The geyser formation mechanisms, air discharge characteristics, and air-water energy evolutions were analyzed. The results indicate that the entry and subsequent release of an entrapped air pocket into the vertical shaft trigger an air pocket-driven geyser, followed by the occurrence of a rapid-filling geyser. Depending on whether these two processes operate independently or interactively, the geyser process can be classified as either a separated type or a hybrid type. A higher dimensionless pipeline pressure difference P * or a smaller dimensionless initial air pocket volume V a* is more likely to trigger a hybrid geyser process. The air pocket discharge ratio during the air pocket-driven geyser is approximately 2%-47%; it first increases and then decreases with increasing P *, and it decreases with increasing V *a. The air pocket discharge ratio during a rapid-filling geyser is approximately 0-5%, and it is minimally affected by P * and V *a. During the geyser process, the peak kinetic energy of the air occurs in the air pocket-driven geyser stage, while the peak kinetic energy of the water in the vertical shaft occurs in the rapid-filling geyser stage. The geyser intensity decreases with increasing P * and increases with increasing V *a. An air pocket-driven geyser occurs when the dimensionless maximum kinetic energy of the air per unit mass exceeds 1.5; a rapid-filling geyser occurs when the dimensionless maximum kinetic energy of the water per unit mass exceeds 0.5.