Due to the short wavelength of millimeter-wave radar, azimuth sampling ambiguity is more likely to occur under certain Pulse Repetition Frequency (PRF) conditions, and the imaging quality is greatly affected by the estimation of the Doppler center frequency. To address this, a method for estimating the Doppler center frequency by combining inertial navigation information and echo envelope symmetry matching is proposed, which can accurately estimate the Doppler center frequency for millimeter-wave Synthetic Aperture Radar (SAR) imaging, thereby achieving the goal of stable and continuous image generation. Firstly, inertial navigation information is employed to resolve azimuth sampling ambiguity, and then the proposed echo envelope symmetry matching method is adopted to achieve precise estimation of the Doppler center frequency. This method has high estimation accuracy and low algorithm complexity, making it suitable for different frequency band radars and various time-frequency domain imaging algorithms, demonstrating good universality. The effectiveness of the algorithm in this paper is verified through continuous estimation of SAR image sequences and performance comparison of the algorithms.