Abstract: To suppress the commutation failures of the line-commutated converter based high-voltage direct-current (LCC-HVDC) system, the static synchronous compensators (STATCOM) are used in the receiving system. However, with the development of LCC-HVDC system and the construction of new power system, the receiving grid is weakened. Under the weak grid, the interaction between the LCC-HVDC system, the STATCOM and the weak grid may cause oscillations. This paper adopts the impedance method to analyze the interaction stability between the LCC-HVDC system with the delta-connected cascaded STATCOM and the receiving grid. Firstly, an improved calculation for DC voltage and AC current of the LCC is proposed to avoid the linearization of the commutating switching function. Then, based on the harmonic state space (HSS) theory, the multi-input multi-output (MIMO) impedance models of the LCC-HVDC system and the delta-connected cascaded STATCOM are derived. Besides, to facilitate the measurement and stability analysis, their MIMO models are converted to a single-input single-output (SISO) model. Based on the SISO model, the impedance characteristics of the LCC-HVDC system with and without the delta-connected STATCOM are analyzed, which reveals the oscillation mechanism of the LCC-HVDC system with the STATCOM in a weak grid. The analysis shows that: when the LCC-HVDC system operates individually, the receiving system is prone to the positive-sequence harmonic oscillation in the middle- and low-frequency band; while after the delta-connected STATCOM is connected, the receiving system is prone to the negative-sequence harmonic oscillation in this band. Finally, the simulation is performed to validate the analysis.