The article discusses the use of mental math exercises in forming symbols about sets, numbers and counting for 5-6 yearold children in preschool. The research results show that psychology is the basis for forming mathematical symbols for preschool children such as: quantity, numbers, counting, space, time, shape, size. However, at each preschool age, the content, form, and method of formation are different with different requirements based on the development of children’s thinking and suitability to the development characteristics of each child. The research results also demonstrate that if there is a suitable direction of impact such as using mental math in preschool mathematics education, it will improve the effectiveness of child care and education.