In 1939, Chen Li began to study the law of intellectual development in response to Spielman's G-factor invariance theory (at least from 9 years old to 18 years old). He compiled 10 test, tested nearly 800 students in primary schools, junior high schools, senior high schools and universities (equivalent to the age range of 9 to 18 years old), calculated nearly 400 related data and conducted complex factor analysis. The results show that the composition of factors is simplified with the increase of age, which has important theoretical significance. His paper "Factor Analysis of a Group of Tests with Different Educational Levels" was published in the American Journal of Developmental Psychology, which attracted great attention from international psychologists. Anastasi, a famous American psychologist, first quoted it in his book "Differential Psychology". Later, in his Psychology of Human Differences, Taylor thought this article was a staged paper on the development of the G factor. In his book Intelligence and Experience, Hunter confirmed Chen Li's conclusion and J Piaget's "conservation" theory, from uneven level to uniform level. In fact, Piaget's theory is similar to the conclusion in Chen Li's paper, that is, the factor model is gradually simplified. Whether the development of intelligence follows the way of differentiation or integration is still inconclusive, but Chen Li's research and conclusion are undoubtedly the beginning of the intellectual development problem that is still being hotly debated.

These theoretical studies of Chen Li established his early position in the field of international psychology.

1947, Chen Li published the article "Correction Formula of Pairing Test" in the American Journal of Educational Psychology, which greatly corrected and simplified the complicated calculation formula of pairing test proposed by Shen Yougan and Zubin. He thinks that the premise of their formula is not to look up knowledge, but to randomly choose answers from ignorance, so it lacks the consideration of validity. In addition, he saw that students had great difficulty in using this formula in teaching, which led to the motivation to simplify the formula. Therefore, he proposed a revised and simplified formula. This fully reflects his persistence in teaching. As the first psychologist who introduced and applied factor analysis in China, Mr. Chen Li always felt that he had the responsibility to control the test well and prevent the abuse of test statistics. To this end, he published many articles, such as My Views on Testing, Misuse and Abuse in Conventional Statistical Methods, Theoretical Analysis of Test Validity, and Preliminary Evaluation of Item Response Theory, which had a good influence on the correct application of psychological testing in China.