To investigate the geyser phenomenon characterized by sudden water-air eruptions and localized pressure surges during the filling process of water conveyance systems, an experimental system for transient water-air two-phase flow was designed and constructed. By utilizing the water level difference between upstream and downstream water tanks to indirectly control the base flow, the water-air two-phase transient process during the filling of a water conveyance system equipped with a ventilation shaft was investigated, aiming to explore the triggering mechanisms and dynamic characteristics of geyser phenomena. The experimental results indicate that the geyser transient process considering base flow can be divided into the air pocket impact stage and the eruption stage. During the air pocket impact stage, the air pocket undergoes repeated compression and expansion, accompanied by periodic pressure fluctuations. During the eruption stage, violent water-air interactions occur, a higher base flow rate results in less gas entrainment into the riser, and geyser phenomena are more likely to occur if the entrained gas arrives at the top orifice of the riser later than the free water surface. The geyser phenomena can be classified into four types: multiple-eruption type, no-eruption type, single-eruption type, and damped-eruption type. Additionally, the propagation and superposition of pressure waves induced by air pocket rupture and coalescence during geyser events are identified as significant contributing factors to the unstable pressure fluctuations.