A novel birefringence measurement method based on a ptychographic iterative engine (PIE) in a planar polarimeter is proposed in this paper. The complex amplitudes formed by a sample in a dark field under two different polarization states are reconstructed using the proposed PIE method. The phase retardation and azimuth angle are simply and accurately extracted from the probe phase and ratio of two complex amplitudes, respectively, and the two-dimensional quantitative measurement of birefringence samples is realized. A birefringence resolution target is used to verify the proposed method, and the obtained results are completely consistent with the resolution target's actual distributions. The maximum phase retardation error is no more than 23.9 nm, and the azimuth angle error is 0.49°. This method has a simple structure and enables a traditional planar polarimeter to quantitatively measure birefringence. The proposed method reduces the number of required PIE scans and shortens the data collection time and processing process, providing a practical method for the birefringence measurement of large-aperture optical devices.