Cryptic strabismus is a condition in which the visual axes of both eyes have a tendency to deviate from each other, but can maintain monocular vision in both eyes through normal fusion function adjustments. In the clinical presentation, the patient’s strabismus symptoms are sometimes manifested and sometimes hidden and not easily detected. During animal evolution, the eyes of mammals have gradually shifted medially to a state where they are essentially parallel in front. However, due to residual lower animal developmental features, the orbital axis of both eyes in humans maintains a bilateral separation. In addition, because the macula of the retina of both eyes is deviated from the posterior pole of the eye, the visual and optical axes of both eyes do not coincide, and because the development of both sides of the face and the orbital material are not absolutely balanced, the vast majority of people need to adjust through binocular fusion mechanism to maintain the same target in both eyes. The fusion function refers to the neurological reflex movement that restores the binocular position when the object image is shifted from the corresponding point in the retina of both eyes through the regulation of the visual center of the brain. Once this fusion function is disrupted, a physiological separation of the visual axes in both eyes is called cryptorchidism. Since physiological causes of cryptorchidism can be found in more than 90% of normal people, some people consider mild cryptorchidism to be normal while complete orthophoria is considered abnormal. Given the prevalence of cryptorchidism and the fact that it can produce severe visual fatigue when fusion is reduced, it is important to understand the classification and perception of cryptorchidism. Cryptopia can be classified into four types: internal cryptopia, external cryptopia, superior cryptopia and rotational cryptopia according to the direction of eye position deviation. According to whether the eye movement is restricted or not, it can be divided into two types: co-rotational and non-co-rotational occlusion. The non-congruent strabismus with limited eye movements is mostly related to mild paralysis of the eye muscles, and the clinical so-called cryptorchidism is congruent strabismus. Strabismus not only affects the patient’s appearance, but also affects the patient’s visual development, and if left untreated, it may lead to the loss of stereopsis. Therefore, it is important to be vigilant.