During early operation period of ±800 kV Xinsong-Dongfang UHVDC project, the construction progress of the accessory power plants at the sending-end falls far behind the original schedule, causing a lack of generation problem in sending-end power grid. In such a situation, the stability of the islanding system after Xinsong station switching into islanding mode, the overvoltage of the islanding system and the harmonic resonance risks of the islanding system are key elements which affect the transmission capability of Xinsong—Dongfang UHVDC project. All of these elements are analyzed in this paper. The analysis results show that the transmission capability of Xinsong—Dongfang HVDC project is constrained and unable to meet the demand of system commission tests. For enhancing the transmission capability of Xinsong-Dongfang UHVDC project, a stopgap according to which the UHVDC system will block itself following a preset strategy if the converter station switches into islanding mode is presented. And the transmission capability of Xinsong—Dongfang UHVDC project after adopting the stopgap is also analyzed. Finally, the quitting conditions of the stopgap, is researched under which Xinsong—Dongfang UHVDC project can reach its rated transmission capacity safely without any stopgap. The study results of this paper guarantee the security and adequacy of Xinsong—Dongfang UHVDC project’s power transmission in early operation period, and also provide valuable guidance on experiments conducting in system commissioning test periods.