Abstract: Aiming at the complex topography and geological conditions,the mass fractured rock by fissures cutting,the poor integrity of strong weathering zone which is easy to form landslides,collapse,unsymmetrical pressure,debris flow and other problems in Xiaba reservoir,after the reconnaissance trip,combined with< Specification for design of hydraulic tunnel building>（SL386-2007）,（SL279-2002）,this paper takes overall consideration the engineering land occupation,the construction difficulty and the engineering investment and other factors,optimal chooses the water intake and drainage scheme of reconstruction the diversion tunnel as the pressurized water taking／venting tunnel which can realize three function by one tunnel.From the hydraulic calculation,the tunnel lining calculation,the stress stability check and others analysis,the results showed that the structure,flow capacity,crack width and stress stability of the tunnel buildings can meet the standards requirements and the optimization design scheme has higher technical feasibility and economic rationality.