Abstract: When the LCC-HVDC system is connected to the weak AC system, there exists strong interactions between the rectifier and inverter station. This paper finds out that in weak AC systems, under different combinations of rectifier-and inverter-side AC system strength (featured by short-circuit ratio(SCR)) of the LCC-HVDC system, with varying constant DC current controller parameters, which are dominated by the rectifier-inverter interactive oscillation mode, the whole system stability behaves as the 'monotonicity- nonmonotonicity' alternation variation characteristic. Then, by means of damping ratio characteristic curve and parameter sensitivity analysis, the mechanism of 'monotonicity-nonmonotonicity' alternation phenomenon is investigated. The results show that this phenomenon is caused by the rectifier-inverter strong interactions under the mutual restriction of the AC system strength at both sides, which reflects the 'barrel effect' of the system stability, that is, the stability moving trend of the whole LCC-HVDC system is dominated by the converter station whose AC system strength is weaker. Finally, the PSCAD/EMTDC simulation results verify the above 'monotonicity-nonmonotonicity' alternation phenomenon of the stability moving trend of the rectifier-inverter interactive oscillation mode. This study can provide theoretical support for having a command of interactive oscillation mode in the practical engineering and selecting reasonable control parameters.