A high-precision reconstruction algorithm is proposed for ptychographic iterative engine (PIE) with highly tilted illumination. By replacing the highly tilted illumination with exiting light field of vertical illumination and modifying the optical transfer function in free space, a computing formula suitable for calculating the large-angle light propagation is obtained together with the corresponding iterative reconstruction algorithm. This proposed method successfully avoids the under-sampling of steep phase ramp of light beam on a sample plane while ensuring the computing accuracy for the diffraction intensity of the non-paraxial light beam on recording plane, overcoming the most significant technique problem that hinders the further improvement in reconstruction accuracy and the applications of single-shot PIE in various research fields.