The invention relates to the multi-terminal flexible direct current power transmission (VSC-HVDC) field and particularly relates to a control system and a control method for a multi-terminal flexible direct current power transmission system. According to the method and the system, based on power requirements of an alternating current side of converter stations and constraint conditions of a converter and a direct current line, lowest operation power consumption of the whole multi-terminal system is taken as an aim to calculate an optimal power flow of a direct current network, and thereby common reference voltage of the direct current system, power instruction of each converter station and sagging coefficients required by gradient control are determined. N-1 verification of a converter station of the multi-terminal direct current power transmission system is realized by employing the control system to calculate multi-terminal direct current network power flow, carrying out analysis and making control protection strategies; N-1 verification of the direct current line is realized by employing the control system to calculate multi-terminal direct current network power flow, carrying out analysis and pre-making control protection strategies, so flexible and safe control on power flow in the multi-terminal direct current network and direct current voltage can be realized.