To clarify the influence of large scale new energy source like wind power and photovoltaics on power system stability, the mechanism of transient voltage stability of new energy source with power electronic interface is studied. The simplified model of new energy source in electro-mechanic transient is first studied. Variable speed wind turbines and photovoltaics with power electronic interface can control the power quickly and flexibly. In power system electro-mechanic transients, the fast dynamics of power adjustment can be neglected, and the new energy source characteristic is simplified to power balance algebraic equation. The system dynamic equation is a differential algebraic equation. According to the stability theory of differential algebraic systems, the trajectory meeting a singular point of the differential algebraic equation corresponds to transient voltage instability in the system. The power balance equation of the new energy source becomes unsolvable at the point. Simulation results validate the mechanism, and reveal the variation of instability mode of a wind-thermal-bundled sending system. As the wind power increases, the instability mode of the system changes from rotor angle instability dominated by synchronous generator to voltage instability dominated by wind power.