The training method for the phase image generator includes the following steps. Firstly, in each iteration, a loss value is generated, including: (1). the phase image generator generates a plurality of generated phase images using a phase image generation mode; (2). the phase image determiner determines a degree of difference between the generated phase images and original phase images; (3). the loss value of the generated phase images is generated according to the degree of difference. Then, a selector selects a stable loss value from the loss values, and uses the phase image generation mode in the iteration corresponding to the stable loss value as a selected phase image generation mode of the phase image generator.