When the grid frequency is varied, the traditional single-phase Phase-locked Loop (PLL) using DFT becomes dissatisfactory. To solve the problems an approach is proposed where ellipse fitting algorithm is incorporated a single-phase Digital Phase-locked Loop (DPLL) based on DFT considering frequency variation. Recursive least square method with forgetting factor is adopted to fit and identify the ellipse parameters, and sine and cosine signals are generated and output by phase-locked loop, then a DPLL on the basis of two-phase orthogonal voltage vector for single-phase digital phase-locked loop is implemented. The influence of parameter variations of the grid voltage including frequency fluctuation, phase mutation and harmonic injection is simulated by MATLAB. Finally, the experimental results verify that the approach is feasible and has better steady-state performance and dynamic characteristics.