A three and a half year old child with a visual acuity of 0.5 is a normal phenomenon. Three and a half year old children, relatively speaking, is still relatively young, visual function is still in the process of development, so visual acuity 0.5 is a normal phenomenon. According to the law of children’s visual development, the lower limit of the normal value of visual acuity for children aged 3-5 years is 0.5, and the lower limit of the normal value of visual acuity for children aged 6 years and above is 0.7. Usually, you should pay special attention to the common sense of the eyes, do not use the eyes for a long time, reading time more than 20 minutes should be rested for 3~5 minutes to look away. You can also do eye exercises, increase the time of outdoor activities, outdoor activities must let the child look at the green or look away. If the three and a half year old child vision loss, it is recommended that the patient go to the hospital in time to check, to avoid adding attention to fatigue caused by strabismus or amblyopia.