Abstract: In order to study problem of load sharing in combined bearing of primary support and secondary lining in composite lining structure of farmland water conveyance tunnel，design of composite lining structure of a pressureless water tunnel in Lincang City，Yunnan Province was taken as an example，according to initial support and secondary lining structure.Internal force calculation of composite lining structure was carried out with load sharing ratio of 7∶3 for lining，and rationality and feasibility of adopting 7∶3 primary lining and secondary lining bearing ratio in design of composite lining structure were discussed.The internal force of secondary lining structure in composite lining structure of unpressurized hydraulic tunnel was analyzed in order to provide certain guiding significance and reference value for design and construction of the same type of water conveyance tunnel.