The　stability　of　Modular　multilevel　converter　(MMC)　itself　is　the　premise　of　analyzing　the　stability　of　MMC　cascading　with　other　cells,　so　this　paper　addressed　the　small-signal　stability　of　MMC　with　dc　voltage　control　mode　which　is　the　common　operation　mode　of　grid-side　MMC　in　high-voltage　direct　current　(HVDC)　systems.　First,　the　small　signal　model　of　MMC　with　open-loop　control　mode　and　dc　voltage　control　mode　while　considering　the　dc　voltage　controller　and　current　controller　are　established　with　the　harmonic　state-space　(HSS)　modeling　method.　Subsequently,　eigenvalues　and　participation　factors　are　analyzed　to　estimate　the　stability　of　MMC　system　and　the　influence　of　controller　parameters,　especially　the　proportional　coefficients　of　dual-loop　controllers,　which　can　improve　the　design　efficiency　of　control　system.　Finally,　time-domain　simulations　in　MATLAB/Simulink　are　provided　to　verify　the　accuracy　of　HSS　small　signal　model　and　the　effectiveness　of　the　theoretical　stability　analysis.　©　2020　by　the　authors.