[Objective] In response to the call for “dual carbon” and to promote the consumption of renewable energy， a mid-long term optimal scheduling of cascade hydro-photovoltaic-storage system considering matching DC transmission with receiving-end load and frequency safety constraints is proposed. [Methods] Firstly， targeting hydropower retention in the sending end and the demand for peak shaving demand of load in the receiving end， an optimized regulation mode for DC external power transmission is constructed. Furthermore， considering the frequency safety of system operation， with the pursuit of minimizing system operation costs and penalty caused by abandoned solar， a mid-long term optimal scheduling model of cascade hydro-photovoltaic-storage system is proposed. On this basis， to address the uncertainty of photovoltaic output， a two-stage distributionally robust optimization model based on comprehensive norm constraints is proposed. The first stage optimizes the system scheduling decisions in the basic scenario， while the second stage minimizes the penalty for solar curtailment and solves it through column and constraint generation algorithms. [Results] Case validation shows that： optimization modeling of the power transfer mode of the DC contact line can fully invoke the coordinated optimization potential between the DC outgoing transmission and the generating units at the sending end； the larger the proportion of hydropower retention and the PV penetration， the smaller the dependence of the system on thermal generating units， and the smaller the demand for peaking at the receiving end； considering the frequency security constraints in the dispatch model is conducive to the prevention of frequency overruns in the wake of accidents； using the distribution robust optimization method to model the problem considering PV uncertainty can obtain optimized dispatch decisions with both safety and economy in mind. Distributed robust optimization method is used to model the problem considering the uncertainty of PV， which can obtain the optimal scheduling decision with both safety and economy. [Conclusions] The proposed mid-long term optimal scheduling strategy of cascaded hydro-photovoltaic-storage system can adapt to the current trend of high penetration of new energy， and the distributed robust optimization model can take into account both economy and security， so as to cope with the challenges brought by the uncertainty of new energy prediction to the power system scheduling in the actual power grid.