The personal direct income of higher education investment can be expressed by calculating the lifetime income difference between people with higher education and those without higher education. The simple model for estimating the lifetime income difference is as follows: Formula 2: I represents the lifetime income difference between people with higher education and those without higher education; B represents the income difference between people with higher education and those without higher education in the first year; In order to simplify the calculation, it can be replaced by the average annual income difference between college students and high school students after they join the work, where t represents the lifelong working years of the educated after graduation and r represents the annual interest rate.