Transient voltage stability has become one of the important threats to the safety of DC receiving power grids. Real-time online dynamic reactive power source management and control is an effective means to cope with this challenge. Therefore, it is necessary to study the dynamic evaluation of dynamic reactive power demand of DC receiving power grid, and quantitatively evaluate the supporting role of dynamic reactive power equipment on grid transient voltage stability. In the past, it mainly relied on the time domain simulation method to analyze the transient voltage stability analysis of large power grids. It is difficult to realize online application due to the long time consumption of calculation. In response to this problem, this paper proposes an online evaluation method for dynamic reactive power demand of DC receiving power grid based on Support Vector Regression (SVR). Firstly, the STVSI-SVR model which can quantitatively evaluate the transient voltage stability of the power grid is established, and then the dynamic reactive reserve evaluation algorithm based on this model is proposed. Compared with the traditional time domain simulation based analysis method, the algorithm can quickly calculate reasonable results and meet the needs of online applications. At the end of the paper, the effectiveness of the algorithm is tested on a practical receiver network model in China. This work is supported by Science and Technology Project of China Southern Power Grid Ltd. (No. GDKJ00000060).