In the canonical theory, Type Ia supernovae (SNe Ia) originates from the white dwarf experiencing mass accretion from its companion and nuclear burning, which explodes when the mass reaches the Chandrasekhar mass limit. However, the progenitors of SNe Ia and their physical nature are still unclear. In this work, new CCD photometric observations on V617 Sgr and WX Cen are carried out in order to detect the orbital period change and their secular evolution. For V617 Sgr and WX Cen, the rates of orbital period changes were calculated to be Ṗ = +2.14 × 10^-7 day/year and Ṗ = -5.15 × 10^-7 day/year. The corresponding timescales of the period changes are ˜ 0.97 × 10⁶ year and ˜ 0.81 × 10⁶ year, respectively. Large variations of orbital periods plausibly indicate the existence of high mass transfer. They are important examples in understanding the mass transfer mechanism in the supersoft X-ray sources. Accordingly, the white dwarfs may evolve into SNe Ia within a million years in the Galaxy.