Based on the method of characteristics for hydraulic transient analysis in pressurized water conveyance systems, both simplified and refined mathematical models were derived to characterize the hydraulic behavior of a corridor-shaped water-diversion surge chamber with bifurcation layout upstream. Corresponding computational models for the hydraulic transition process of the water conveyance and hydropower systems were established, with a focus on investigating the surge wave characteristics, local flow regime, and their impacts on regulation guarantee calculations under asymmetric operation conditions. The computational results show that compared with the simplified model, the refined model can accurately describe the hydraulic characteristics of the corridor-shaped surge chamber. While obtaining the extreme values of surge waves and parameters for regulation guarantee calculations, the refined model precisely captures the attenuation characteristics of water level fluctuations in the surge chamber under asymmetric operation conditions. Under asymmetric operation conditions of the units, significant longitudinal flows occur within the corridor-shaped surge chamber, which may affect the hydraulic oscillation characteristics and stability of the system under certain conditions.