To realize safe and reliable start-up of a hybrid cascaded DC transmission system in series with line commutated converter (LCC) and parallel modular multilevel converter (MMC) valves on the inverter side, a start-up control strategy based on the two stages division of uncontrollable charging and control unlocking is proposed. First, the mathematical model of the hybrid DC transmission system is established. Based on the analysis of the transient characteristics of the uncontrollable charging stage of the MMC at the low-voltage end, the equivalent calculation formulas of the MMC maximum start-up impulse current and pre-charge time are deduced. Then the selection basis of current limit resistance in the MMC start-up process is provided according to the maximum impulse current and pre-charge time. Second, from the unlocking of the system controller to the steady-state operation stage, the unbalanced start-up current problems between MMC parallel groups when the controller is unlocked are analyzed. Then an optimized cooperation strategy based on the control sequence coordination of different converters and adaptive MMC power reference valve is proposed. Finally, PSCAD/EMTDC simulation results show that the proposed start-up scheme can effectively realize the smooth start-up of a hybrid cascaded DC system. This work is supported by the National Natural Science Foundation of China (No. 51977183).