The　instability　issue　of　the　grid-connected　Voltage　Source　Converter　(VSC)　dominated　by　phase-lock　loop　(PLL)　is　investigated　using　the　electrical　torque　method　in　this　paper.　The　system　dynamic　model　and　steady-state　relationship　of　variables　are　first　established.　Then,　a　basic　two-order　model　representing　the　PLL　natural　oscillation　mode　is　extracted.　The　impacts　of　the　current　control　loop　and　altering　current　grid　(ACG)　on　the　PLL　oscillation　mode　are　derived　using　the　electrical　torque　method.　As　a　result,　analytical　expressions　of　the　system　synchronous　and　damping　coefficients　are　obtained.　Based　on　the　derived　synchronous　and　damping　coefficient,　qualitative　evaluations　of　the　impacts　of　the　system　parameters　on　the　oscillation　frequency　and　system　stability　can　be　achieved.　It　is　drawn　that　a　small　PLL　time　constant,　low　infinite　source　voltage,　heavy　load,　reactive　power　absorption,　and　long　electrical　distance　will　deteriorate　system　stability.　Moreover,　a　sufficient　analytical　stability　criterion　for　the　grid-connected　VSC　is　derived　from　the　damping　coefficient.　The　system　stability　can　be　directly　evaluated　through　the　operation　points　and　system　parameters,　and　the　quantitative　evaluation　of　the　system　stability　is　achieved.　The　eigenvalue　analysis　and　electromagnetic　transient　simulation　have　verified　the　reliability　and　effectiveness　of　the　theoretical　analysis.　©　1986-2012　IEEE.