The effects of core error in scanning and lensless flexible optical endoscopic phase imaging based on coherent fiber bundle（CFB）were studied. Utilizing wave optics theory, a theoretical model has been developed for scanless and lensless flexible optical endoscopic 2D phase imaging based on CFB, addressing transmission mode and core length errors in CFB. The results show that the imaging resolution is different between single-mode transmission and multi-mode transmission, and the phenomenon of super diffraction limit resolution exists in multi-mode transmission. Core length error has a serious impact on imaging. When the core length error is large, imaging cannot be done. When the core length error is small, the imaging signal-to-noise ratio decreases. Wiener filter can be used for fuzzy images with low signal-to-noise ratio affected by small core length error. After several iterations, the signal-to-noise ratio of images can be increased by more than 5 dB, and the normalized root mean square error of images is less than 0.5, which effectively improves the image quality. Therefore, in order to achieve imaging in the scanless and lensless flexible optical endoscopic phase imaging system based on CFB, it is necessary to either use single-mode CFB or to suppress multi-modes in multi-mode CFB, thereby ensuring single-mode operation.