Abstract: As the scale of DC lines continues to expand, energy centers are gradually developing sending end systems capable of delivering significant amounts of power through multiple DC lines. Aiming at the problem of DC blocking overvoltage in multi-send HVDC systems, this paper sheds light on the mechanism of dynamic rising overvoltage phenomenon from a two-time scale perspective and demonstrates the leading role of synchronous generators at the voltage rising stage using singular perturbation theory. Accordingly, this paper proposes a slow system model for analyzing the overvoltage during the blocking period and gives an equivalent static load model of DC during the overvoltage rising stage. It is proved that during DC blocking period, the system predominantly inducing the rising voltage is a monotonic system and the order-preserving properties between overvoltage and generator dynamic parameters are revealed with monotonic system theory. Finally, a fast analytical decoupling calculation method for dynamic rising overvoltage is proposed. The accuracy of the analysis result and calculation method can be validated by the multi-sending end power grid in Ningxia.