A coupling effect analytical method based on reactive power flow is brought up to analyze the coupling effect in a multi-infeed direct current transmission system.An account is given to the mechanism of voltage dip in all subsystems when one subsystem suffers from reactive power disturbance through changes in the reactive power flow.The reactive power supporting coefficient（RPSC）is brought up as a steady-state coefficient to quantitatively analyze the reactive power supportive effect between subsystems.The expression of RPSC is also derived.Based on the above-mentioned coupling effect analytical method，the expression of multi-infeed voltage stability index（MIVSI）in the double-infeed direct current system is derived.The expression and derivation process reveal the relationship between RPSC，multi-infeed interaction factor（MIIF）and MIVSI.The influencing factor of RPSC is analyzed by referring to the expressions.Eventually，simulation in double-infeed direct current transmission system in PSCAD environment is carried out to validate the theoretical derivation.