This paper studies hydration stages and phase transformation mechanism of fresh calcium sulfoaluminate (CSA) cement. Features of different hydration stages of CSA cement paste are identified by non-contact impedance measurement (NCIM), X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transforms infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), heat evolution test, ion chromatograph and inductively coupled plasma. Ettringite (AFt) and monosulfate (AFm) are found as metastable phases and have poor crystallization degree at the early-age hydration. The unique transformation between AFt and AFm is observed. The influence of different w/c ratios is further studied by NCIM, setting time and compressive strength test.