In multi-infeed HVDC system, because the electrical distances among DC converter stations are relatively short, the interactions and mutual influences between DC systems or DC system and AC system are complex. Multi-infeed interaction factor (MIIF) can effectively reflect the interactions and mutual influences. MIIF currently is mainly solved by simulation method which physical meaning is not clear. Therefore, a more realistic Jacobian matrix analytic expression is proposed. The simulation results have verified its effectiveness. At last, three MIIF analytic expressions based on circuit theory and the proposed Jacobian matrix analytic expression are compared with the simulation method, and some feasible suggestions from the practical engineering perspective are offered.