Single wavelength microscopic interferometry, driven by the Piezoelectric Transducer (PZT), is a common surface topography measurement method. Its measurement accuracy is directly determined by the original phase acquisition precision of every pixels in area array CCD. Traditional phase identification methods adopt 3 points or 4 points algorithm to obtain the phase. However, they require the displacement step, actuated by the PZT, to strictly keep the same to satisfy the 90° phase condition. Therefore, these methods are only suitable for the strict anti-vibration experimental environment or conditions with high precise closed-loop PZT actuator and strict calibration of interferometric wavelength. An auto-acquisition driving step method for the single wavelength microscopic interferometry is presented in this paper. Firstly, interference sequence diagrams, containing the surface topography information, are gathered by the CCD under open-loop PZT actuating. Next, two pixels whose phase difference is approximate 90° are selected as the calculating center to obtain smoothed gray values with regional gray average algorithm, which can reduce the influence of random noise. Finally, an optimal fitting algorithm for the ellipse, formed by the average gray values, is proposed to obtain the amplitudes and offsets of the two gray values array. According these fitting parameters and gray values, the drive step can be calculated by elliptic equations. Experiments have shown that this method can reduce the requirement conditions of measurement conditions and improve the measurement accuracy.