In the upgrading of line commutated converter-high voltage direct current (LCC-HVDC) projects, the flexible reconstruction scheme of receiving conventional converter station is safe and feasible. Based on ±800 kV UHVDC project, a series hybrid UHVDC system is formed by replacing the double low-end converters on the inverter side with double VSC converters. In this paper, a voltage distribution strategy for high-end and low-end converters considering reactive power compensation is proposed to solve the voltage allocation problem of series converters in hybrid UHVDC system. This strategy considers the scheme that VSC and AC filter provide reactive power together. By establishing the functional relationship between rated capacity of VSC and rated power of LCC, the voltage distribution results of LCC and VSC are determined after optimization. Based on the electromagnetic transient model, coordinated control strategy is designed and compared with the transient response characteristics of conventional UHVDC system before reconstruction. The results show that the flexible transformation scheme improves the resistance ability to commutation failures of UHVDC system, and UHVDC system under voltage allocation strategy performs good operation characteristics.